Ok now that's just really freaky...you have to time a pause perfectly to see this and yes it does match Thancred's choice of weapon but it's so odd because in the cutscene you don't see this until this split second frame. It's clearly on his left side (Thancred keeps his there) but in the screenshot I posted, there's clearly nothing. No belt to even hold a dagger up, so maybe this is a clipping issue and only in this split second the character's wardrobe under it can be seen? If it is I can see the story tellers checking out the older version of the story and giving us what we have now with Lahabrea taking over Thancred...which we are told happens when Thancred despairs in losing Louisoix. So maybe originally Thancred was going to betray everyone then the changes happened and the new story tellers realized Thancred was popular with the ladies so it was 'best' to make him betray everyone only by being taken over cause he was sad cause Louisoix was/is gone...man really reaching here but that's a nice little find, you have good eyes for seeing that in the split second it's shown lol.

As for thinking it's another body in the coffin and not just empty, it's because of Nielle's shock. If he opened it and it was empty I believe the reaction would have been confusing not a "this cannot be" as he stares into a blank coffin. The reason I say Corguevais is a possibility is because Nielle knew what he looked like and Corguevais was part of that storyline so I don't see it as someone random. The Lalafell twins said they "took care of him." So what would be more shocking: an empty coffin or one with Corguevais in it. But of course the "it's impossible" line to me means it was someone highly unexpected and even Corguevais may not have brought out that kind of response as Nielle could probably care less if he was gone/dead I'm sure. I really don't know but yeah where would Warburton or his corpse be then?