We ate there last week and had a PS for 7:20 and were finished before the fireworks started. (We weren't staying for them anyway.)
But the hostess told us that even if we were finished before the fireworks, that since we had dinner there that evening, we were welcome to come back later for the fireworks.
So if you finished early (or wanted to eat earlier), you could go down to the 4th floor and hit the shops or hang out at the bar at the Outer Rim Lounge, or go down to the big arcade on the 1st floor. Then go back up to the 2nd floor (where you check in for CG) and they will let you go back up.
(Or they should - that's what they told me.)