From what you've written the OW's pregnancy does sound very suspect. But, until you know for sure, I think you should proceed as if she is pregnant. You don't want to allow yourself to hope she isn't pregnant (or that the baby isn't his) and then have to go through emotional hell when you find out she really is pregnant with his baby.
It is great your H wants to come back but make him prove he is doing it for the right reasons. You seem to be off to a good start. I'm sending some positive vibes your way just in case.
