I read a crazy theory that I wouldn't put it past them to use. But someone suggested that Capaldi's character from 'Fires of Pompeii' could really be the 12th Doctor keeping an eye on 10 for whatever reason.
Probably not, as timelords can sense each other.
I think he remembered the look of the Pompeii guy and modeled his new body after him. Like he did with Colin Baker
Probably, that's pretty much the theory I'm going with. Although really, I just imagine that the doctor randomly thinks about his time in Pompeii, and that dude from FOP happens to be on his mind for some reason, and Bobs you're uncle. Same with the 5th doctor regenerating into the sixth.