In the hacked emails Amy Pascal specifically mentions Peter Parker, not Miles Morales. That and the success of the previous Spiderman films with Peter Parker lead you to believe that Peter will be Spiderman in this crossover into the MCU.
As far as Sony selling Spiderman back to Disney/Marvel? I don't see it happening. They've seen what a cash cow the MCU has been for Disney, and it looks like they're far more interested in licensing Spiderman back to Marvel studios. And I think if Fox were presented with a similar option they might consider it as well. Fox still owns the rights to the FF and X-Men. But if Fox sees that Sony's partnership with Marvel is profitable? Then they might consider a similar licensing deal. The Marvel Cinematic Universe has been really successful, and if the studios that bought other characters can cash in on that success too, I think they might be enticed to get into business with Disney/Marvel Studios.
Coincidentally, when all of this Spiderman news was coming out I saw an article on Business Insider
that shows which studio has the film rights to certain Marvel comics characters. The Submariner is owned by Universal, while Fox has X-Men, the FF and some related characters like Wolverine, Cable, Deadpool and the Silver Surfer.