I described the situation as the murderer and his gang/allies, IIRC without scrolling back. So the fight isn't over that quickly. We're talking about an elaborate, exciting fight that is there to bring the story of the adventure to an dramatic conclusion. It is still a big boss battle.
Everyone in the party will use their action points and dailies in this scenario. Everyone will Nova. That's okay, since everyone can "Nova".
But if not everyone can nova, than those that can will dominate this fight, because the novaing character can use a more powerful ability each round, abilities that he would normally have to spread across many encounters that day, he can alll focus in one, leaving no need for the normally balancing actions where he cannot cast a powerful spell but is relegated to firing his cross-bow or using some other weak at-will ability.
There is the imbalance.
And if you do not want to see that, okay. I think this is the last time I'll try.
If its a big boss battle as you said then spending you xp budget for the day will indeed balance the encounter