Cycle: Core Set
Interrupt: When your opponent plays an event card, exhaust a unique Eldar unit to cancel its effects.
No clarifications for this card.
No. Dire Mutation's ability is an interrupt, and therefore triggers before the unit is able to complete the exhaustion. Another unique Eldar unit must be exhausted to pay the cost of Nullify.
Yes. It has been confirmed by Apoka that this is intended.