Hello Catie! Welcome to the Disney Parks Moms Panel.
For a recent Adults-Only experience at Epcot, my wife and some friends made Advanced Dining Reservations for
La Hacienda de San Angel to coincide with Illuminations: Reflections of Earth.
The meal was fantastic and the indoor experience of
Illuminations: Reflections of Earth made for a truly magical experience that I will never forget.
As far as your Advanced Dining Reservations are concerned, if you inform your server that you are looking to stay for the Fireworks Spectacular, you should be fine as long as you are alright with a 2-hour dining experience. I have never been asked by Cast Members to rush a dining experience, let alone for one that includes covered seating for a Fireworks Spectacular.
Having said that, I recommend that you try to get later
Advanced Dining Reservations in order to maximize your time in Epcot as a 2-hour meal may be a little longer than you are comfortable waiting for the show to begin.
With a 9:30pm start time for Illuminations: Reflections of Earth, obtaining Advanced Dining Reservations for between 8:15pm and 8:30pm would be ideal timing to be just finishing your meal and savoring dessert as the Fireworks begin.
Have a Magical Evening!
~Todd