Which of course means if the way had been cleared Mab cleared it.

So four came and were detected by the web.

Just to make the record clear this all occurs before the event's of the Fool Moon Garage which means the Scarecrow was already here.

The Scarecrow was at the Fool Moon garage roughly the same time that Molly was being abducted.  Harry's web spell detected the fetches, Harry sends them to the fear beacon Molly, and Harry gets abducted by Madrigal Raith after polishing off a stray fetch at the convention.  Madrigal takes Harry to the garage where the Scarecrow attacks while the other fetches abduct Molly from the Carpenter home.

Mab definitely allowed Harry and company access to Arctis Tor, but that didn't require her to sacrifice her elite troll guards.  I don't think you're saying this, but the Arctis Tor attack doesn't seem to be Mab's idea.  More like something where she outmaneuvered someone else.

My theory has always been that Molly was the focus of PG because someone knew (or believed in the possibility) that she would one day become the Winter Lady. The fetches were intended to draw Harry's attention to Molly's use of black magic, because without him, she would have certainly been caught, tried, and executed. The main whole-series outcome of Proven Guilty is Molly becoming Harry's apprentice, setting her on a path to replace Maeve as Winter Lady.
Personally, I find it convenient that the first fetch assault accomplished three things: it sealed up Pell's theater, attracted Harry's attention, and framed Molly's... boyfriend? Side piece? I don't know what the cool kids call it these days. Molly was a beacon, and so was the guy (because he was the target of black magic and had the scent of fear that attracted them).
In my opinion, the Arctis Tor attack succeeded at the intended objective: clearing the way for Harry's party. Harry, Thomas, Murphy, and Charity barely had enough in them to handle the fetches; if Mab's elite bodyguards were still kicking, they almost certainly would've failed.
The players we know about:
1. Mab
2. Maeve
3. The Arctis Tor Attackers (speculated in the books to be Thorned Namshiel; at the very least, it appears to be someone capable of using Hellfire)
4. Molly
5. Lily and Fix
6. Nemesis
7. Rashid
Other potential players:
1. Uriel (or other agents of TWG)
2. A third party who may or may not be misleading the Fetches

...

Etc., etc. Basically, if Molly bites the dust and Harry throws down with the Council, everything goes down with them. So it's my belief that someone (likely someones) knew she was going to be important, and helped push the attack on Arctis Tor to make sure things worked out as they did.

Molly is definitely the focus of PG, and I think that time travel shenanigans were definitely involved.  Here's my current theory with some unanswered questions.

Assumption:  The last book before the BAT will be the time travel book because every detective story needs a reveal near the end to show the audience the answer to the mystery.  The mystery in the Dresden Files is figuring out the actions and goals of the Black Council/Circle.  To give the readers that information, Jim will need to put Harry's eyes on key events that were impossible in the earlier books, hence the time travel.  It's also nice fan service to give dead characters another appearance.
To extend this assumption, Harry is traveling back in time and working in the background of several case files books, notably PG for this portion of the theory.

Harry goes back in time in order to preserve his timeline.  Standard time travel rules say you shouldn't meet your past self, so he works only in the background of stories.  PG gets started when the Gatekeeper gives PG!Harry a message to look out for dark magic.  Bob explains that messages like that have to be intentionally vague or risk changing the timeline by themselves.  My theory is that future!Harry gives Rashid the message to send to himself, and Mab becomes aware of future!Harry.  They're both at the Outer Gates.  future!Harry is also the Winter Knight, meaning Mab can make definitive statements about Harry becoming her knight without lying (which fae can't do).  It also gives Rashid information about future!Harry one day going against the Council that he mentions later in TC.  This also motivates PG!Mab to send fetches at future!Harry's request to get the first attack on Pell and PG!Harry's attention. 

future!Harry is trying to keep Molly from becoming a full-fledged warlock without changing his timeline.  Sandra Marlin gave her the idea to use fear as a motivator (suspicious, but maybe innocent) that set her on the road to warlock-dom.  If no one intervenes, Molly is going to go to the dark side where no one can help her.

----
Edit:  Forgot to add the car crash at the beginning of PG. 
If no one intervened, Harry would have used a broken Little Chicago to search for dark magic and killed himself.  Instead, future!Harry gets in an old car and wrecks PG!Harry to delay him enough that Molly's phone call interrupts his preparations.  The oldness of the car is the giveaway that it's a wizard driving.
----

Here's where the theory gets a bit murky.  Mab sends the first fetch attack on Pell (similar warning tactics were used by Mab in SmF with the Hobs and the Archive).  We also know that Maeve reveals her Nemesis infection to Mab in PG based on Mab being so upset she loses her voice in later books.  That means that Maeve has to be an active player in PG that bumps her elbow or that future!Harry tells her.  I think the first is more likely.  I put the second fetch attack on Maeve sending the fetches.  This way, she can confuse Mab's warning to PG!Harry. 

Questions that still need answers:
1)  Glau the jann and Madrigal Raith - Who set them up to be a cutout / fall guy?  How did they know to do that?
2)  What are the details with the third attack?  Did Mab send the three fetches to grab Molly (or did Harry really redirect them like he thought)?  Who sent the Scarecrow / Eldest Fetch?
3)  Arctis Tor:  Who were the attackers?  What was the attackers' goal?  Did they succeed?
4)  Why didn't Mab work with Summer and allow both faerie courts to make the Red Court pay for intruding on their territory?  This is likely related to the Arctis Tor attack.

Opening the field to time travel shenanigans (which should be done, these are time-travel specific Outsiders), you can't really narrow down the suspects to just the list of characters we saw in PG.  Time travel allows it to be anyone that fits the magical prowess criteria.  People like Cowl and Kumori are fair candidates for our future time travel book occurring behind the scenes of our other case file books.

With that in mind, there's no reason to contort ourselves into knots trying to shoehorn in a PG character as the summoner.  We don't yet have the summoner's motivation in order to summon cornerhounds nor proof of its intended target.

The Mirror Mirror book isn't the time travel book.  Mirror Mirror will start with Evil!Harry summoning an alt-universe version of himself (our Harry) as a way to fake his own death.  Evil!Harry's reality was created by a choice of Harry's in GP by WoJ.


For that choice, I think one near the end of the book works best, though several would probably fit the bill.  As an example, the "I love you" to Susan when he's in the basement with her after being captured.  Swap that choice, Susan goes feral and either forces Harry to kill her or she kills Justine.  The first turns Harry into a full warlock in the WC's eyes and the second will mean Thomas never becomes an ally.  Either way, his life is totally different from that moment on.

A slinky is a circle when looked at along its axis or rotation and thus any figure that can be inscribed within will work.  I wish this was an original idea but helical spirals fit the idea of time travel very well and I've seen it in the past somewhere. Maybe here.

I referred to the flat version. As I have stated to Mira, I don't guess when there is text and I suppose I am obsessive compulsive to that point.  The text says 13.

Check out covering spaces in topology if you want to see more about the idea of a helix projecting onto a circle mathematically.  That kind of thing inspired Ms Duck's pancake theory of reality years ago.

The text says 12 before it says 13.  ¯\_(ツ)_/¯  Either way, I doubt it'll be relevant story-wise.

As opposed to Mac's Place where everything adds up to 13, and if we are all correct, Mac is an ex-angel if not an angel.

That's a good point.  No reason that the 13 can't be significant for multiple reasons.  Non-angelic + magically dispersive.

Jim has mentioned before that he drew inspiration of pentagrams in multiple dimensions from one of these.  At least, something along those lines.  The D&D nerd is coming out in him.

Alfred is more than a mere Genius Loci,  he is an Intellectus..

Turn Coat page 278 Harry tells Morgan about his contact with Alfred and here is Morgan's response.

There is a hell of a lot more to it as Morgan goes on to explain to Molly, but it's getting late and I don't feel like copying another page and a half.

Alfred has intellectus about the island, yes.  The prison being built allowed him to ascend to a higher place than your typical genius loci.