I'm not really sure I get the "what if Enterprise could still fire" concept. She was fully operational, and then Vengeance tapped her on the butt and she curled up and took a beating. Given that Marcus is the highest admiral in SF, he knew Enterprise in great detail. I very much doubt the initial shots by Vengeance were just random pew-pew in universe. Take out the engines, take out the weapons, and then go to work violently dismantling the hull.
Even if the Enterprise had weapons, at that point, with all the damage, i'd venture to guess their wouldn't have been enough power for them to matter. Vengeance just outclassed her. which really makes me curious what Vengeance would have been like against a Klingon Armada, or if the Klingons have their own version of the Vengeance (Sword of Kahless from Klingon Academy in the next Trek, anyone?

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I mean, I would assume Vengeance has a really good shield grid, to say nothing of it's really armored looking hull. We don't know enough. Could conventional phasers even do any damage to that hull plating? I mean, really, how much of a game changer is the Vengeance?
Oh, that is one element I really want to see explored more, too. The idea of a starship with drone launching capability. It's such a "now" inspired notion, and it makes perfect sense. Who needs a fleet when you have a large C&C vessel that can deploy flying phaser and torpedo platforms to do the work of a small task force for you?