Rulings
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.
(wotc • 2024-02-02)
If you can't exile enough cards to meet or exceed the required mana value, you can't choose to collect evidence at all.
(wotc • 2024-02-02)
Rulings
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.
(wotc • 2024-02-02)
If you can't exile enough cards to meet or exceed the required mana value, you can't choose to collect evidence at all.
(wotc • 2024-02-02)