Anyway, the Old Ones were eventually defeated locked up in prisons in reality, although some may have been cast out. We don't know enough about that time to be sure. Presumably star born helped with this, and it all seems to do with the Cycle (a loop of events that includes the 666 year conjunction that creates star born and some other event where the balance of power is reset and/or the Outsiders get a good chance to come in). Some call it the Game.

I'm not sure if we have a firm year for the Dresden Files, but 666ish years before DF gives us a bunch of really, really important events.
1. The Renaissance kicks off; Dante's Divine Comedy, Botticelli, etc.
2. The Black Death wipes out anywhere from 50-200 million people, upwards of 1/4 of the world's population (which sounds awfully apocalyptic, and I'm pretty sure Nicodemus was involved; it could very well have been hijacked from Outsiders).
3. Probably not relevant right now, but this is around the time the Aztec empire really started (it's around this point that one of the main city-states, Tenochtitln, was established; it's unclear when exactly it was founded, but 1325 is often the cited date). Basically, this is around the time that we start having a clearer picture of what the various Mexica city-states and civilizations were doing. Expansion, war, sacrifice, etc. I don't know how relevant this might be in the future, because the only thing close to the Aztec we've seen in the series are Kukulcan and the Yucatan after they were hijacked by the Red Court.
4. The Hundred Years War kicks off. Joan of Arc isn't around until much later (1412 is her birth year, so she's likely not a Starborn, though it wouldn't shock me in the least if she was a Knight).
5. Chaucer basically decides that English is a good enough language to write in. This sounds like less of a big deal, but The Canterbury Tales in particular basically established Middle English as a legitimate literary language, which kind of shunted aside Italian, French, and Latin. Fun fact: Chaucer pretty much created 2000+ words. Shakespeare only managed about 1,700. Suck on that, Bill.
6. 1376: The Bible is translated into English. Huge, HUGE deal. John Wycliffe almost certainly did most of this himself, and it's almost 100% that he personally translated John, Mark, Luke, and Matthew, but it's possible he was responsible for translating the entire New Testament. The King James translation doesn't come for 230ish years. The main point here is that Wycliffe helped set the stage for Protestantism in the 16th century.
7. Around this time (137...8? 1379?) you have The Great Schism, and two rival Bishops both claiming to be the Pope. This lasts until somewhere around 1417, I think (I'm a medieval guy, and most of my interest ends around the Magna Carta, so someone please correct me if I'm wrong here).
8. Late, may or may not be relevant at all, but Tamerlane initiates the last Mongol Conquest in 1398ish. A Sunni Islamic culture, this was pretty much the last of the Nomadic Eurasian Steppe empires (the Timurids). For those wondering, the Empire included Iran, Afghanistan, Turkey (parts), some parts of India, Central Asia, etc.
I know it's easy to look at any century and say "Hey, this was a period of upheaval!" That said, the 1300's really WERE a period of TREMENDOUS worldwide change. The Black Death, the initial translation of the Bible, and yes, Chaucer legitimizing English as a literary language are all really, really important.
The Black Death didn't quite kill Feudalism, but it sowed the seeds of doubt among the peasantry and is often credited with helping that process along. In fact, one of the results of the Black Death was Parliament passing a LOT of laws mandating wage freezes, which helped cause a big Peasant Revolt in the 1380s (also helped that taxes kept  going up to fund the Hundred Years War's various ongoing conflicts. People really hate taxes. If any of you ever become a dictator, seriously consider cutting taxes. Tax hikes usually don't end well for people in charge, while tax breaks are almost always well-received). Feudalism also ties into my ongoing William the Conqueror-was-a-Starborn-and-Hastings/Stamford-Bridge-was-the-last-time-control-of-the-Outer-Gates-changed-hands theory, since he's the one who really brought it to England with his implementation of vassalage.
Anyway, the point is that the biggest major event 666-ish years ago was the Black Death, and that really, really altered the course of Western Civilization for centuries. I don't know if a Starborn was involved, but I think it's entirely possible.
Also worth noting that Vlad Tepes's father, Vlad II Dracul, is said to be born in 1395. Since we know he's a Starborn, it's unclear exactly how close the 666-year cycle lines up; that could just be an incorrect birth year that was given because he's immortal, but it might not be. Celestial movements may not need to be precise; there may be a length of time during which Starborn can be created, and there are other things that contribute to their status at birth (I like to think there is a birth ritual involved somewhere).

Jim really likes employing common tropes and putting a spin on them, so I am looking forward to his take on a therapist conversation, Freudian Couch and all.

I'm tempted to label this "time travel shenanigans." In fact, when this particular scene was released several months before Peace Talks came out, I read the description and the term "cornerhounds" and immediately said "Hounds of Tindalos in Dresden?"
When Butters opens up the very next chapter with "Holy crap, Hounds of Tindalos are real?!" It solidified my opinion that time travel would kinda have to be involved in some way, else it'd probably be a different type of Outsider. It feels too much like an obvious clue/reference.
My argument:
1. Justine is not the summoner, because she needs Harry to live long enough to escort her to Demonreach, Nemesis's real goal. It does not suit her plans to try to kill Harry, especially not when she knows exactly how to manipulate him. (Literally just "being a woman asking for help" is enough 99% of the time for Harry). She also does not demonstrate any wizard (or even warlock)-level talent in any previous book. Thus far, the only non-wizard/non-warlock we see using mortal-style magic is Thomas, when he uses the locator spell Harry taught him (offscreen in... White Night, I think), and Butters, when he makes a circle in Dead Beat (I'm excusing all of the single-use magical items he uses in his pseudo-Batman impression, though maybe there's more than a little talent required to make those). My understanding is that you do need legitimate magical talent to summon an Outsider.

2. Time and effort are required to summon one, and even then it is dangerous for the summoner. Harry does a full ritual whenever he does anything of the sort. In Proven Guilty, there's a definite spell that builds energy to summon the Fetches. I don't think Justine had the time, and Eb is onscreen long enough before they show up that I don't think he did it, either.

This leaves the wizards we know are in Chicago, but aren't in that scene: Carlos, Chandler, Listens-to-Wind, Wild Bill, Yoshimo, Martha Liberty, and Cristos. Potential additional summoners (those who have enough power) include: Molly, Mab, Titania, and Sarissa. I don't think the Queens are mortal enough anymore to summon an Outsider, but Molly and Sarissa probably are. Odin and Ferrovax are out for the same reason. The Archive could do it; so could Marcone, I presume, though I find them unlikely.

Based solely on character history, I'm going to eliminate several: Listens-to-Wind, Wild Bill, and Yoshimo. LtW doesn't strike me as the type who'd do business with things that upset the natural order, and Wild Bill and Yoshimo get carried off by Drakul, presumed Dead or Turned.

That leaves us with Carlos, Martha Liberty, Cristos, and Chandler. It's possible that Carlos did it, and there is one interesting potential possibility: Carlos was tracking Harry at the time, and knew where he'd be.

Martha Liberty is a relatively unknown quantity. She's only had a handful of lines in the series, most of them supportive of Harry. She's also an odd one among Wizards: she lives with her descendants.

Cristos is pretty obviously a bad guy, but he also fights pretty hard in Battle Ground. Earth magic, it looked like. It's possible he's a stooge.

No, of the wizards in Chicago during Peace Talks, IF the cornerhounds were indeed summoned, my money is on Chandler being the culprit. We know a handful of things about Hounds of Tindalos that may (or may not) apply to cornerhounds: they are drawn to time travel. Chandler's specialty is messing with time. It may or may not have been intentional, but Steed is the only wizard explicitly stated by Jim to have an affinity for time-related magic, and now we have time-travel-related Outsiders show up.

3. Related to my last point, I think we're left with only two possibilities: A) the cornerhounds were indeed summoned, to kill Harry. My bets are on Chandler being the one responsible, though I concede Justine is possible, just not with any of the skills we can confirm she possesses (meaning it'd be a thus-far unseen ability of hers or Nemesis's). I would imagine those particular types of Outsiders would be the first ones to answer Chandler's summoning, mostly because of the aforementioned affinity with time.

The other possibility is the one I find more likely: B) the cornerhounds are there as a side effect of time travel happening behind the scenes. Either it's a time travelin' Harry on an adventure we'll see detailed in a future book, or it's Chandler. Remember, we have no idea what happens to Steed when Drakul tosses him through a portal. He may have used time travel magic to escape, or that might have been what Drakul did to him in the first place. Based on Odin's explanation of time travel (and the ripples on the past) during Cold Days, it's possible that someone bending the laws of time a few hours after the attack (during the Drakul fight) might have an impact several hours earlier, so I think it's possible that the cornerhounds showed up earlier due to such a ripple effect.

I reread (well, re-listened, actually) Peace Talks and Battle Ground recently, and I don't think it's ever explained where they came from. A mortal had to summon them, right?
Except they're the Dresden Files iteration of the Hounds of Tindalos, which are creatures that find and hunt you when you mess with the fabric of time.
So, questions:
1. If they were summoned, who summoned them? Again, it needs to be a mortal practitioner. Could it have been Justine? My understanding is that magic was necessary to summon an Outsider, and Justine isn't magically talented. Justine is the obvious answer, particularly because they were attacked right as Harry left. But Justine needs Harry to take her to the Island, so why risk that he'd be torn to pieces?
2. Was there time travel during the events of Peace Talks and Battle Ground? I find it intriguing that it was specifically Cornerhounds who were summoned. Why those particular types of Outsiders rather than any of the others we've seen? They weren't especially effective at attacking Eb and Harry. Were they intended to fail, or were they following their instincts to attack time travelers?

Like I said, it's entirely possible the answer is simple "Justine, duh," but I think there's some room for debate here. Anyone have any thoughts?

I'm not sure Mac could have completely transubstantiated and still match with what we've seen in the books. When Uriel lent his Grace to Michael in Skin Game and started bleeding, there's no mention of the cut healing, and I think Uriel has to stem the blood.
I'm firmly in the "Mac was an antediluvian Watcher who Fell because he sired at least one child with a human" camp, but you do raise an interesting counterproposal. It does indeed seem strange that Mac was brought to Demonreach. Harry's connection with Mac is pretty damn loose. It'd be like kidnapping Jason Bourne's barber, wouldn't it? Of course he'd try to get him back, but it's not like Harry's relationship with Mac is extremely close or anything. I've at least never gotten that sense.
Molly, Carlos, Murphy, Susan (pre-death, obviously), Maggie, Michael, Thomas, Butters, Luccio... all of them would make better motivating targets to use against Harry. Mac's on the list, but he's somewhere near the middle at best, in my opinion. And Justine alone would've been plenty to use as a hostage. Sarilla makes sense too, for the reasons you mentioned—good to have the backup vessels at hand, and that's all shown clearly.
So why Mac? There's something to it, I completely agree.
I see two possibilities that can fit. First is your theory: I could be totally wrong, and Mac indeed used his Grace to help build Demonreach. "Raphael" would fit, at least in terms of the name's meaning, which literally means "God heals," (Hebrew origin) so that meshes well with the one supernatural ability Mac's demonstrated on the page. "McAnally" means "Son of the poor man" (Gaelic origin), if anyone's interested.
To continue in this vein, I'll ask an absurd question: what if Demonreach (the genius loci itself/Alfred, not the prison or the Island) is Mac's old Grace? When Lasciel and Harry had a "baby," it became a spirit of Intellect. Angels themselves are specifically mentioned as possessing Intellectus during the explanation in Turn Coat:

"Intellectus," I said. "Um. It's a mode of existence for a very few rare and powerful supernatural beings—angels have it. I'm willing to bet Mother Winter and Mother Summer have it. For beings with intellectus, all reality exists in one piece, one place, one moment, and they can look at the whole thing." (Turn Coat, page 281 on Nook).

The interesting part to me is that Intellectus is described in two completely different ways in the same paragraph. I underlined and bolded the part that's relevant to me: if it is indeed a "mode of existence" (rather than something an entity possesses), that indicates, to me, that Intellectus is a state more than it is a capability. Harry's experience with it is one that is shared, not something Harry himself has.
So in this adjacent theory, I think you're mostly right, but I'd posit that the Demonreach entity was born of Mac's Grace, rather than something he lent to Merlin (the way Uriel lent his to Michael). That very well might have been what allowed Merlin to construct the prison across time; because he was working with Grace, which (going by Harry's words) is a mode of existence that experiences time all at once, all he had to do was form some kind of thaumaturgic connection between the Grace and the physical location. That connection would need to be strong, but not necessarily ultra complicated, which is kinda how Bob describes the runes Harry has him examine, if I recall correctly—the complexity comes from them overlapping across time, I think, not from anything native to the wards themselves.
Possibility number two goes back to Mac being one of the Watchers. The Book of Enoch states that the angels who sired the Nephilim Fell, and were "bound in the valleys of the Earth," until Judgment Day.
Wild theory: what if the prison of Demonreach was the location in which those Fallen angels were bound? I posited in another thread that 8-10k years ago, Chicago was the bottom of a lake, carved out by retreating glaciers, and that Chicago may very well be the valley described Enoch. Demonreach itself is even further below Chicago (Chicago Under Chicago, maybe, the opposite to Chicago Over Chicago depicted in Summer Knight). Mac, a Watcher, sired a Nephilim with a human, and in order to avoid permanent bondage in Demonreach, chose to transubstantiate. His Grace instead was caught and imprisoned there. It's also hypothetically possible in this wild theory that "out" literally means "out of prison," meaning Mac escaped Demonreach, maybe by leaving his Grace behind as ballast (the way Santa stays behind in Cold Days while Harry and the Wild Hunt head to the Island). It's possible there'd be a reaction if Mac is injured so close to his imprisoned Grace, or maybe the "attack" on the prison was somehow using the connection between Mac and his Grace.
Well, anyway, I'm at least glad we can think about this for real rather than throwing out WMGs about what Mac even is. I remember someone claiming (maybe on here, maybe elsewhere; it was many years ago) that Mac was the original Mother Summer, who abdicated. When it's obvious that that title belongs to Mrs. Spunkelcrief.

It's not just the bodies—that's more a problem for Harry than Lara. The problem is that his Hunger is so starved that it's turning against Thomas and is consuming him. Putting his Hunger to sleep may pause that consumption process long enough for Thomas to feed.

It was also not a unified belief with a holy book and so on. Local variations were all equally valid. Some gods were more popular than others with some people and so on.

Big time. There's almost as much variation within cultures as there is between cultures, and that includes religions. I mean, even monotheistic religions have significant variations between sects and denominations, and even within the same sect or denomination you'll find plenty of differences from church to church (or temple to temple, or parish to parish), and that's with written documents that ostensibly unify beliefs.

It's in the books that it's one time, but there was a WOJ that massive personality changes might allow a second. Big enough changes are extremely rare though.

I recall that WoJ, but I've also always related that to Names, too. Harry states more than once that a mortal's Name isn't all that usable, because humans constantly reinvent themselves. Makes me wonder why a Name is more mutable than the circumstances allowing a second Soulgaze.