And here's the transcription of the actual trailer since the closed captioning sucks. This isn't true/full captioning with descriptions, but it does have the spoken dialogue.
Well it wouldn't be an actual treekiller as he's from Illian (nine bees on chestplate). But everyone else did come to support them, so that doesn't rule out your theory.
We had a bit of a commotion in Yellow chat. It could be one of the Sitters we see in the Hall of the Tower, but otherwise we don't think it's a named character.
Working theory is that's a funeral b/c everyone but Whites wear white at those. Plus they're upset. Rafe did an exclusive trailer...