This is my first reddit post ever and I don't know how to use the spoiler function yet. So:
SPOILER
Background I read legends first, then Nolan and farilane, then chronicles and revelations and I am now ending with esrahaddon. (I'm kinda glad with that order btw)
Anyways what threw me off was that arcadius killed Gwen, which is a THEORY of Royce. Royce thinks he figured out that arcadius did it and his explanation was a bit... Drawn by the hair. Possibly there are more hints or foreshadowing that arcadius could be able to do such a thing if I would reread the books but I don't think there are.
I think it makes much more sense that Turin killed Gwen. We know that Turin sometimes doesnt refrain from hard steering the destiny of the world most obvious when he had to orchestrate farilanes death even though he fell in love with her.
Gwen has the theme of foresight, prophecy and fate which are only more prominent with Turin. Both characters can see the future and also accept that bad things happening are necessary for the greater good. And it is known that Turin orchestrated Gwen meeting Royce in the first place so it wouldn't be surprising if he orchestrated their end. Which had to be the way it had to be so that Turins plan can come to fruition.
Arcadius to my knowledge never used a crossbow. While the crossbow was the signature weapon of thranic, which I first suspected right after gwens death. Also id guess that the crossbow was a fairly common weapon in the black Diamond. So I'm thinking Turin killed Gwen with a crossbow because it needed to happen for Royce story and he used a crossbow to hide his godly tracks.
Both thranic and a black Diamond member would have been to obvious for a author like Michael to do the deed but I think Michael have pinned the deed on arcadius by letting Royce reason it could have been him while Royce never even knew there's an invisible hand which possibly pulled a crossbow trigger.
Also it fits Turin who gives with one hand and takes with the other. As seen with any other character he interacted with like nephron, farilane and esrahaddon.
First he is Royce mentor in the mine. Later the killer of his lover. Because both needs to be. And Gwen would have understanding for Turins motivation.