Or, y'know, he could shatter to little pieces like the "keyblade" that he held. Why do you think the female knight was so desperate to catch him? As he fell, his mask was further smashed, but other than that, it appears as if he suffered no serious physical disfigurement.
I think it makes a lot of sense that the enigmatic knight dude (or whatever the hell they're calling him now) is Xehanort. Especially given the scene in Final Mix+ featuring a discussion between Xigbar, Zexion, and Vexen about Xehanort and the keyblade war that occured before Xehanort's appearance at Radiant Garden. Also, at the beginning of that scene, Xehanort goes down into what seems to be the "Room of Sleep" where he looks at armor that looked vaguely similar to what was worn in Sunset Horizons.
I think it would be an interesting twist to have Xehanort turn out to be the hero of the next game, especially since we have relatively no clue what happened to him before he lost his memories.
As for the Roxas look-alike... until there's more evidence suggesting any relation to Sora (I don't put much stock in vague comments Nomura makes when he tries to dodge questions), I'm merely going to assume that the resemblence is simply because, eventually, Nomura's characters all start to look frighteningly similar.