How would McCain become Prez?
Everything that I read at the time of the election said that if the election winner were disqualified, the election would go to the runner-up. That WOULD have been McCain.
Of course, this situation is unprecedented. While it should go to the runner up (IMO), he has been sworn in for nearly 16 (horrible) months. It would probably take the rest of the term to litigate the true successor... :-(