Instant
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Behind the Mask will overwrite any previous effects that set a creature or Vehicle's power and toughness to specific numbers. Effects that otherwise modify its power and toughness will still apply no matter when they took effect. The same is true for +1/+1 counters.
If Behind the Mask causes a Vehicle to become an artifact creature, it doesn't count as "crewing" that Vehicle for any ability that would trigger off of a Vehicle becoming crewed.
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.