Pepper_W,
Too bad you felt like you had to implement a rule like that to get the job done. But I maintain that there was no need to do it. The rules are in place for it already.
Player A is first to show. If he only turns one card over then the other players in the hand simply wait for him to turn the other one over before giving him the pot. If player A continues to delay the game by only showing one card and expecting others to show two before he does then you give him warnings and penalties. Done, show over. You don't have to complicate things further with the one card forward motion stuff. But I am glad it worked for you in the end, of course.
BTW, the need to show both cards to win a pot is a secondary condition that exists only if there is another hand looking to claim the pot. The "last live hand" rule is a primary condition. A hand is absolutely able to win a pot without showing (unless there is contention for the pot)
