Our most recent visit to CG was in January, one of the slowest times of the year. We waited 50 minutes past our ADR time before we were seated and we had arrived 15 minutes early which put our total wait time at 1 hour, 5 minutes.
They will probably let you go up to the restaurant so that you can watch the fireworks from the outside viewing platforms but I wouldn't count on getting a table in time. Anybody who is sitting at a table that close to fireworks time isn't going to get up and leave. Also, there is very limited seating in the lounge area while you wait for your table. We have found that most often, we stand in a fairly congested area right off the elevators while we wait for our table.
Also, I'm sure you know already, there is no guarantee that you will get a window table anyway. There are only so many of those and everybody wants one.
The fireworks are nice from here but not the greatest view. From inside the park is much better. Personally, I would not go to Ca Grill for the intention of watching the fireworks. Sure, it's a nice perk if you hit it right but I would only go because I wanted to eat there.