Two players,one all in. All five cards are dealt and face up. The winning player shows his hand on top of the mucked cards face up. He did not say fold, but showed his winning hand. Is that a muck?
In tournament play, the hands should have been face up before the board was dealt, and players cannot fold, so this shouldn't happen. In a live game, if a player's hand is clearly tabled face up, it plays. The fact that it may have touched the discards or the board is not relevant. There might be a casino with specific rules to the contrary, but I've never encountered one.
It all depends on the casino rules. Judging by what you said, he knew he had the winning hand so it was pointless to fold it. I'd say it's not a muck.
It depends on the casino rules. But if the cards are mucked (meaning the cards are in contact with other rest of the mucked cards), the cards are considered "Dead". But if the cards are shown first, and then it touched the muck, some casino will rule as Shown cards allowed to win the pot.
Both timing & contact with the mucked cards will be the components to decide whether the cards are dead or live.
In most cases I have witnessed, this is not a muck. They showed their cards after they won and threw them in the center. It is clear that they have the winning hand and in no way would they ever have the intention of folding, unless they explicitly said so.