I do disagree with your rather casual assumption of McCain prevailing in any debates. Even if Obama didn’t have a sincere bone in his body you can bet he would have all his talking points wired by the time he faced McCain on the podium.
We’ve all seen the guy speak and that is definitely his strong suit. They don’t call him the media darling for nothing. Whether Obama’s sincere or not I don’t think there is a politician alive who could just walk over him.
As for the independents breaking NcCain’s way, I think it’s a little early to be counting those particular chickens. The whole reason they’re independent is because they have no loyalties to either of the dominant parties. To me that means that most of them will reserve their options right up until the moment they step into the voting booth.