I want to take the OP and change it a bit. I firmly believe that it's Thomas on the island and not some impostor. However, that doesn't mean there wasn't the use of an impostor for Thomas. (This almost deserves to be it's own thread.) I'm sure some of you know where I'm going with this but I want to start off with some basic facts.
Evanna told Harry that cameras show Thomas making the attack, but Harry hasn't seen that video. Harry hasn't seen the weapons the assassin used. So much happened, but no one outside of the swartalves themselves have seen the evidence.
Harry mentions that Thomas is very good in close combat situations. We seen that at Chichen Itza and elsewhere. What we've also seen is that Thomas is comfortable using firearms and he is especially skilled with the use of edged weapons. I don't recall him using explosives. Maybe someone can find an instance when he has, something I've forgotten about, but it would be an outlier event.
The assassin starts his assault with a explosions, plural. Perhaps as a distraction it's plausible that is something Thomas might have done. We are not certain how the assassin tried to kill Etri, we just know the Austri put himself in the way and died as a result. (Or at least, that's the official story.) So what did the assassin do? Throwing an explosive device, say a grenade, at Etri isn't really Thomas' style. He could have shot at Etri, but whatever type of weapon was used, when Austri went down, why didn't the assassin follow up? However the attack was made, Thomas should have had a blade of some type to follow up his initial attack. We don't know exactly what happened after Austri died, but the assassin was quickly subdued after that because Etri didn't appear to be badly wounded. If Thomas had gotten close to Etri, the Swartalf should have taken some nasty slashing or stab wounds but had none that Harry could see. I find it a little too convenient that Mr. Etri wasn't killed or even badly wounded.
One other fact to mention. Harry mentions that the swaralves allowed their anger to control them and beat Thomas to the point where he could barely speak when they should have been questioning Thomas, trying to find out why he attacked Etri and who he was in league with. What if that wasn't an accident, what if it was part of the plan?
So how do I picture this going down? Thomas was sent a message to go see Evanna immediately. The message didn't have to come from her but other than Gedwig, she's the best suspect. Thomas needs to feed anyway so he swallows the bate and goes. When he arrives, Thomas is jumped and beaten, or perhaps more likely, he's drugged first and then severely beaten. The assassin is a shapeshifter. The attack may not have even been meant to kill Etri. The real assassin is grabbed and dragged away but it's all a show and he is quickly released. An unconscious Thomas is shown to Etri who is told that the assassin refuses to give out any information.
So here's my explanation of why this may have happened as I described. This could be part of a larger game to isolate Harry. Take out one of his most reliable and dangerous allies. Alienate Harry from the people who currently shelter him. Make him more vulnerable. This could be Mab's and maybe even Lara's doing, but it could also be an Outsider / Black Council plot. The reason I mention Lara is I don't recall her being angry or even slightly disturbed by the fact that she has to use her favors to rescue Thomas, rather than use them to accomplish some other scheme she was working on. What doesn't fit, or at least as yet it doesn't fit, is I don't see what Lara would get out of it. I totally understand why Mab or his Black Council enemies might want to isolate Harry.
