You may target a creature that is already tapped with Crimestopper Sprite's last ability. If the target creature is already tapped as the ability resolves and evidence was collected, you will still put a stun counter on it.

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

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.