The Twyleth Teg are probably Wildfae, neither Summer nor Winter nor a member of the Accords. They are hereditary enemies of the Fomor, so this was a score settling, nothing more and the head a message “see our long term enemy who has thwarted us and kept us down for centuries, that’s his head that is, so when we say we can take you all, you better believe it”

Agreed; my suggestion is that Nudd's death was claimed by the Fomor (and by extension, in theory, the Black Council/Outsiders), and this suggests a parallel to the assassination attempt on Etri (unless that was something more convoluted, which might break the parallel) being ultimately sourced in the Fomor/Black Council/Outsiders.

I'm not necessarily suggesting that Thomas is Nemfected; he was likely a patsy.  But his controller would have been connected to Team Outsider, as opposed to various other theories, like "Eb coerced Thomas into attacking Etri as an attempt to get rid of Thomas, in order to protect Harry."

(Something that cuts against the parallel is that I think Mab was aware of the attack on Etri either a bit before or immediately when it happened, but she seemed to be surprised by the death of Nudd.  It doesn't necessarily destroy the parallel, but I thought it was worth noting.)

...And now I have a new theory!  One of the other mysteries were "who controls the cops/FBI that were watching Justine."  ...Has anyone seen the Erlking recently?  Since, you know, two other heads of Wyldfae courts were just attacked, and one killed?

This is the first of Harry’s friends to die in Peace Talks/Battlefield Ground.

Austri's death was announced earlier in terms of page count, but yeah, Gwyn ap Nudd probably got offed before that.

...Which raises the question, how long ago did Nudd get killed, and unless it was "on the way to the party," why didn't anyone else know?  And, now that I think of it, do Nudd's death and the attempt on Etri constitute a pattern?

Another possible angle is that the Starborn breaks a lot of the usual rules by his specialness, and even the wise old mentors don't know where all the limits are, and they want Harry to be able to take full advantage of "I didn't know that wasn't possible, so I did it anyway!"  (Granted, this angle seems a bit out of character for Eb, but also, if Harry had known what Demonreach was, he might well not have punched Alfred in the nose and become the Warden.)

From a Doylist angle, Jim has used this angle in the Codex Alera--without giving specific spoilers, there's a point where a character does a thing, and it's pointed out immediately afterwards that he must have slept through that lecture in school, because the dominant academic theory is that said thing won't work.

But none of that is relevant to the fundamentals of Thomas and Justine's relationship.  Yes, at one time, Justine was protected from Raiths by the power of True Love.  That was an outgrowth of the existing connection between Thomas and Justine, plus some mystically-significant qualifications.  The mystic protection was broken in the GS epilogue (either temporarily, as hoped for, or permanently), but there's been no evidence that this shifted their fundamental devotion to each other.  Thomas lives and breathes for Justine, and the fact that she had True Love protections is proof that it was reciprocated on her part.

Yes, their relationship could have downgraded to the point that Justine no longer loves Thomas, and would willingly betray him to his death.  There is no evidence that this actually happened, however.  We're back in "Harry is secretly a serial killer" territory.

I'm not saying that the True Love angle makes Justine a saint, or definitely inclined to make the best choices, but practically by definition, it must make her loyal to Thomas.  I'll accept theories where she was genuinely deceived, but not ones where she willingly betrayed Thomas.  She's been quite consistently devoted to him.

I think the Thomas + Justine True Love goes a fair way to defending her character, particularly from the angle of being loyal to Thomas.

That said, I've yet to see a theory that resolves the "why exactly did Thomas attack Etri?" question satisfactorily.

Also, how confirmed is it that Thomas was the assassin vs. Thomas being framed?

It depends on exactly how much you trust information from the svartelves.  Thomas plausibly has the means and reason to be present (either visiting Harry or one of his svartelf booty calls), and at the end of the day, the svartelves had physical possession of Thomas.  It's possible that Thomas was framed; either 1) the svartelves are lying about the events that took place (the svartelves are in on the frame job), or 2) someone else did the hit, framed Thomas, and fooled Etri and the rest simultaneously (the svartelves weren't in on it).

I'm inclined to believe the svartelves are being honest--it seems like it would be a major departure from their characterization otherwise--and I'm uncomfortable with the theory that someone snuck past svartelf security and pulled off the assassination attempt while framing Thomas and escaping undetected.  There are probably a handful of characters that could pull that off, but...the intended result would have to be Etri not dying, since I don't think it's plausible that someone could so thoroughly penetrate svartelf security (completely undetected!) and miss their shot at Etri.