Dinner at a signature restaurant typically takes 90 minutes to 2 hours for apps, entree and dessert. Since you are planning just drinks, apps and dessert it will probably be closer to 90 minutes.
If you choose the 7:05 res you will definitely be done before the 10:00 fireworks. You could sit at the lounge with a drink or explore the shops on the 4th floor. You can show your dinner receipt at the 2nd floor check in podium and you will be able to come back up for the fireworks.
With the 9:05 reservation you will probably be finished with apps, then go outside to watch the fireworks and come back to your table for dessert. Let your server know you want to go outside to see the fireworks and they will pace your meal so that you are between courses. We often order dessert, then head outside.