If evidence was collected and an opponent has multiple creatures tied for the greatest power, that player chooses which one to sacrifice.

Once you've announced that you're casting a spell, players can't take actions until you've finished doing so. Notably, opponents can't try to remove cards from your graveyard to stop you from collecting evidence.

Starting with the next opponent in turn order (or, if you cast Extract a Confession on an opponent's turn, starting with the opponent whose turn it is) and proceeding in turn order, each opponent chooses a creature they control to sacrifice. Then those creatures are sacrificed at the same time.

If you can't exile enough cards to meet or exceed the required mana value, you can't choose to collect evidence at all.