A: We would rule that if caught sufficiently quickly in the failure to say “Yeahuh!” the player putting down Keepers would lose at least the second one, if not both. If they played them slowly enough to allow another player to call them out about the failure to say Yeahuh on the first one, then that chance is lost when the second Keeper is played… however, we suspect the Keepers in question were played in quick enough succession that there wasn’t even a chance to catch them in between, in which case we’d rule that as long as they were caught immediately (not, say, as an afterthought. “Oh – wait! You didn’t say Yeahuh!”) then they should be forced to give up those cards, and forfeit the win.

It’s a lot like using a Surprise to cancel a win. Yes, if you play the Surprise in a timely manner, you can reverse even a game-winning play.