Where I have played (Minnesota, Wisconsin, Nevada) the situation you describe would never happen in a cash game (and would be very unlikely to happen in a tournament either).

When a player is all-in, they rarely flip their cards over. In most cases, they don't flip their cards over unless they win the hand (otherwise they just muck).

Even in the cases where they choose to flip their cards over before the rest of the hand plays out, the dealer would never grab both hands and bring them to the center of the table. The hand would remain in front of the player.

However, even if the dealer did bring both hands to the center of the table, the hand still belongs to the player and they have the right to muck their hand whenever they would like to.

It's really quite simple. If a player wants to muck their hand they can (at any time) and once the hand hits the muck, it's dead.