From my experience, having dined at CG multiple times, you will not be seated by the time the fireworks start. Imagine, you are sitting at a table, you are done with your meal but the fireworks are starting in 15, even 20 minutes. Would you leave the restaurant? People linger, hang onto those tables, especially if they have a decent view.
So the seating backs up worse and worse the closer it gets to fireworks time. You may get lucky and be seated by 8:55 but the chances are greater that you will not. Our last visit here, we were seated 50 minutes past our ADR time. The good news for you is that the place rapidly clears once the fireworks are over and you will have a much quieter meal than if you book earlier in the evening (this place is noisy).
What will more than likely happen is that they will allow you up into a packed lounge area that will be standing room only (as long as the place is not already at capacity limit) and you will be allowed to out onto the observation deck to watch the fireworks.
In answer to your other question, they dim the lights in the restaurant and pipe in the music. If you have ordered courses, you can request that your server pace your courses so that you do not have a course delivered right before or during fireworks so that you can watch the show. Probably 1/3 of the restaurant have seats that can see it from their tables, less than that actually have prime seats. A good majority of diners go out onto the observation decks.
I would not be upset about your dinner time. You will be able to see the fireworks from the observation deck and then enjoy a quieter than normal meal here. We now book all of our dinners late like this so that we don't have to deal with the noise.
