No Errata
Q: If my opponent plays a card that pushes more than one fighter (like Vile Invaders) can I play this card as a reaction after any of those fighters is pushed? And if so, which fighter can I push?
A: You can play this card as a reaction after an opponent pushes any fighter, even if they are partway through resolving a rule that pushes more than one fighter. When you do so, you can push any fighter other than the one that was just pushed (the one you are reacting to), even if the fighter you push was already pushed by the rule your opponent is resolving or if that fighter is going to be pushed by the rule your opponent is resolving.
Q: Can I play Mirror Move when a fighter from my opponent’s warband drives a fighter back during an Attack action?
A: Yes.