Can Bringer of Dreams and Nightmares combine Damage from multiple Powers - eg, use a "2 Damage" Power and a "1 Damage" Power to earn 5 Fear for an unhurt City?

Can Bringer of Dreams and Nightmares combine Damage from multiple Powers - eg, use a "2 Damage" Power and a "1 Damage" Power to earn 5 Fear for an unhurt City?

No. First the "2 Damage" Power resolves and does nothing (because that Power would not have destroyed an unhurt City). Then the "1 Damage" Power resolves and does nothing (because it would not have destroyed an unhurt City). Neither has any lingering effect of any kind.
If it were a single Power doing 2 Damage then 1 Damage, it would earn the Fear, because then that Power would have destroyed an unhurt City. For each Power Bringer uses, briefly imagine a universe where you resolve the entire Power normally. Whenever it would destroy an Invader in that universe, in the real game you earn 0/2/5 Fear and Push it if it's an Explorer or Town.