[Veil]

This is very simple. As a dealer, the number one rule in this situation is that to be eligible to win a pot you simply must show your cards. There can never be a case when there is no bet at the final street, where both (or all) players are allowed to muck. Face up cards at the river is an indication of a check unless the player explicitly (puzzling behavior) motions to concede the pot (rules may differ depending on the casino of course). However, as soon as player B motions to muck, it's over. It is Player A's pot, with absolute no question - as long as the face down cards were at least slightly moved towards the betting line. You simply cannot have people pretending to muck their cards and then say "Oh I was only kidding," etc.