Irish Mike,
Contrary to tournament poker, most cash games allow any player at the table the right to see a called hand. There is a local casino that took that privilege away because certain players abused it. This is what happened; during a "cash" no-limit game with blinds of $1 and $2 with a max $100 buy-in, a player lost his third pot by being outdrawn on the river, by the same player. To make the situation worse, after the winner asked to see what the opposing player was so proud of (because of all his aggressive betting during the hand), the player mucked his hand (in anger) and said to the dealer; " Every time that son of a, no good, lowdown, etc, etc, is in a called hand, I want to see it! That is what is known as abusing the rule.
I repeat: Every player at the table has a right to see a called hand. The method for the dealer, if there is a request to see the mucked hand is to kill the hand by touching it to the muck before it is revealed. This is mearly a formality because the hand is already dead after it is surrendered by the owner. The best method is for the dealer to set the cards in question off to the side, award the pot to the winner and then expose the cards so, should the best hand prove to be the mucked hand, it is too late and there is no pot to dispute.
If the declared winner of the hand asks to see the mucked hand and it turns out to be better than his, he (the would be winner) will lose the pot!
Mike, I did want to mention that in your first example the correct order of showdown was not followed. Player A should have revealed his hand before Player B. This does occur on a regular basis, and there is nothing wrong with it. In fact, it is encouraged if the calling player knows he has the best hand. It prevents slowing down the game, but....it is not in compliance with the proper order of showdown. When Player A bets and is called, he (the player who initiated the bet) should turn his hand first.