I hope Zack sorts Alistair out rather than the other way around. Is Alistair going to be welcome anyway with those flashy black wings of his?
Based on how Dark said that Darkwings in the spire were allowed in the spire if you were in the guard... but Alistair seemed to hold power over the guard and the town in general, so he'd probably be allowed back on the basis that he's powerful.
Also if a black-winged light power says he wants something, I generally get the impression that you shouldn't say 'no'.