He plays it cool of course when he returns (because he's the Doctor and he knows how awesome he is), but it was a chance in a million.

Oh, that's a nice way of putting it. I think you're right. And in the same way that it would have ruined it if it had been quite obvious that he would return, it would have been somewhat out of character and too much if he, like Amy, had been all tearful and overwhelmed when he came back. It was much more subtle, the way they showed his joy through his ridiculous dancing and all those long looks he gave Amy and Rory.