Thank you for all the quotes you provided here!

The body switch made Luccio more susceptible to the ink than a wizard of her age should be.
 A few quotes from the book (all are from the paperback version and all italics are my emohasis)...
 From pages 128 to 129:
from pages 518 to 519
from pages 526 to 527
 Luccio is older than Morgan and Eb said in the first quote that Morgan could not have been forced to kill against his will. Luccio was more susceptible because of the body switch and the resulting younger mind. Peabody was able to have a greater influence on her with his ink...

And yet, none of those quotes (nor anything else I recall Ebenezer ever addressing directly) actually states that the bodyswap made Luccio more susceptible to the influence of the inks, and to Peabody's magic.

We infer it, from the 1-2-punch of Morgan telling us of Luccio's guilt and from Ebenezer says.

... I doubt that Peabody could force an older wizard, including the Senior Council, to accompany him to the island. That goes far beyond the nudging Eb mentioned. If a Senior Council member was present on the island, I believe they had to be there of there own will as a co-conspirator, not because they were forced to by Peabody. Both Eb and Harry believe that it was Cristos on the island with Peabody, but we know that they could be wrong.

So, here's some other thoughts; alternative hypotheses.

#1 - Ebenezer is just wrong.  We know that the Council has largely forbidden the exploration of mind-magic, but Peabody & his ilk clearly have not.  Corpsetaker had remarked on how out-of-date Harry's training was, and I think we should take it for granted that Eb gave the best mental-defense training that he could, to his grandson.  But what if Black Council researchers had found ways to bend even Senior-Council level minds, without breaking them?

#2 - Luccio is Black Council.  This is a dark theory, but -- what if Luccio killed LaFortier not because of Peabody's mind-control, but because she's a full-on BC agent?  What if "Peabody made me do it" is just her cover-story?  Consider this:  of all the people in Edinburgh who could have discovered her... who's the one person who would cover for her, take the fall, but also have the skills & power to escape the WC, and the initiative to loop-in Dresden?  How "convenient" for her, that that one person was the one who found her there with the bloody dagger.

#3 - An overt, brutal, full-control takeover may not be possible, or may be prohibitive.  But repeated nudges, year after year?  That exactly what sets habits and patterns in place, which can become self-sustaining:  "You are what you do."  Small incremental changes, that are let to sit and "naturalize" in place... which in turn can form a new baseline, from which you make further incremental changes, set new habits and patterns.

And it's not like the Council was a bastion of loving-kindness, trust, and openness; there were innumerable doubts and grudges and suspicions to exploit, bad inclinations that pre-existed, that didn't so much need a "nudge" to be go badly, as just gently suppressing the inhibitions that might have stopped that bad decision.

How many years was Peabody inking the Council?

... First, we know that Justine was Nemfected and had to be infected by someone ...

Actually, we don't know Justine "had to be infected by someone."  We have no viable nor exhaustive list of Nemvectors; person-to-person spread may not even be one of them!  IIRC, we don't know of any person-to-person transfers.  Lea got Nemfected by Morgana's Athame (not by a "person" but by a "cursed item"), and I think that's the only Nemfection whose provenance we know precisely (tho there's plenty of WAGs about others).

... Maeve being Nemfected a strong possibility given that she almost blew up Demonreach and that was the explicit goal of Nemesis in Battle Ground. She was also the very first target of the Starborn Winter Knight. 

No, we 100% know for sure that Maeve was Nemfected; that's why Mab ordered her killed.  We know it came from Lea, though not precisely how.

Maeve wanted to beat Mab, to outmaneuver her and take away what she valued.  Nemesis gave Maeve some degree of free will, and choice; much more than other Fae.  Unlike all the others, Maeve could lie (not just mislead & misdirect and slice the truth so fine it was almost invisible, but blatantly and knowingly speak an un-truth); Maeve's dying words were, "two plus two is five" (which, when you think about it, is really rather pathetic:  a trivial little nothingburger lie, petty braggadocio... but also 100% proof that she had Something Big enabling her to violate one of the fundamentals of Faerie).

Lea "spread it to Maeve" (Mab's own words) somehow... but Lea still had the Athame, so could easily have been the proximate source of Maeve's Nemfection, as well as Lea's!

= = =

How did Aurora get Nemfected?  We have WAGs, but canon is unclear on anything beyond that it happened.

How did Cat Sith get Nemfected?  He was "taken" somehow (and just how TF do you "take" a critter like Cat Sith, who's competent & stealthy enough to remove elite sidhe warriors without neighboring elite sidhe warriors even noticing??!?), but we don't know the details of actually passing Nemesis to him.

etc.

Looping back to Justine, however...

My best bet is that her Nemfection happened somewhere in the White Court.  There is a LOT of Outsider influence / presence amongst the Whampires, and I'm betting it was "someone... or something" who Nemfected her there, with my first guess being something in the private "Starborn/Outsider Apocalypse" collection of Raith Père, after she began acting as Lara's personal assistant.  (On a slight tangent:  I further WAG that the substantive Outsider problems amongst the Whamps is Mab's "main" motivation in contracting a marriage between her Starborn-Wizard Winterknight, and White Princess Lara).

I don't discount @Mira's theory, though, that Justine was a deep plant:  Nemfected before we first met her, worming her way into Thomas' affections, in order to get to Harry.  This is my own 2nd-favorite theory.
 

IIRC there's some strong hints (maybe even a WoJ?) that there's an Alera / CinderSpires link, but less so for something as close to our "Modern World" as the Dresden'verse is.

But of course nothing's impossible, and WoJ says the Nevernever is virtually-limitless, and explicitly includes all sorts of fiction & fantasy places, e.g. Spiderman lives somewhere in the Nevernever, and Frodo in some NeveneverShire, &c.

Lara understands that, and loves what she is, accepts what she is.. Thomas hadn't fully until the Skinwalker got a hold of him.
Thomas changed and seemingly made a choice, interesting scene at the zoo at the end of Turncoat.  Harry appalled because Thomas told him he enjoyed what he was doing after a while.  Actually Lara was delighted with that outcome and didn't want Harry messing with Thomas on that score..

We never got to see that struggle for the heart and mind of Thomas between Lara and Harry.  Or Harry having to come to grips that his brother was now no different from other vampires now.  I was disappointed that we didn't. 

I don't think the Skinwalker did "convert" Thomas; not really.
It tortured Thomas until he gave up hope in his gentler and more-loving aspirations.

But even then, he wasn't the merciless predator it had tried to create:  he was angry & hurt that Harry still wanted that from him & for him, expected it of him.  He wouldn't have been hurting/angry if some part of him didn't still see that as better!  The Skinwalker had convinced him that he was a broken thing, unworthy of having nice emotions & nice relationships.

... so that leaves Cristos (technically not on the Senior Council at this time) and Martha Liberty.

I think we have a large number of named/identified (but minimally-seen) characters who potentially might be that "unindicted co-conspirator" on the WC... Klaus the Toymaker, the whole list (Montjoy, Lucciozi, Gomez, etc) recited before Ebenezer got named to the Senior Council, and maybe some others I'm not recalling.

Offhand, I don't see any good reason to suppose it's any one of them over another.

Cristos -- specifically -- strikes me as particularly narratively-useful in his very vagueness:  "is he evil... or just stupid?" but also that just-trying-to-get-by everyman.  Despite being a skillful wizard, he's busy with politicking and reputational advancement, trying to "business-as-usual" the problems away.

... I know that it is listed on the Publishers website for 1/20/26, but that seems a little off since we have not had an official announcement from the author or his website ...

I would expect the author to have minimal insight and zero control, at this point; the publisher has the authoritative info.

Jim learns the release date when they tell him.

I think Harry being broke all of the time can also get old after a bit...

It's worth noting that the "financial struggles" theme is pretty endemic to the "noir detective" genre that was foundational to the Dresden Files.  Also, it has been narratively-useful as a way to get Harry involved when his "common sense" suggested he not do so.

On the other hand, he's working directly for a superpower these days.  I think a good argument can be made that we're no longer within the "Noir Detective" genre (even as a fantasy mashup).  Mind you, his sponsoring "superpower" is a remarkably-stingy one, so I think he'll remain short of ready cash in his daily life... even as he owns a castle in Chicago, and a hidden island in Lake Michigan, like some billionaire supervillain would.

Burger King & IHOP (and Pizza 'Spress) will be his "splurge" foods, unless he solves some magical problem for a high-end chef who gives him "always eat here free" privileges as a reward.