We've seen wedding parties before when we've eaten at Victoria & Albert's. The restaurant has a separate area with perhaps 5 tables and a fireplace. Could be perfect for your party.
If you want to eat in Epcot, you could consider Bistro de Paris. Not sure they have a private area, per se, but the restaurant itself has an elegant, exclusive feel to it.
Marrakesh doesn't have a private area that I'm aware of, but the restaurant has always been quiet and half empty when we've eaten there. And it has entertainment, including a lovely belly dancer.
California Grill has a wine room with a few tables, which would feel more private. Doesn't have the view, but we always ask for a table there as it's much quieter and much less frenetic than the main dining room. Also, there is a back room at California Grill that normally seats about 50 -- we've been to Signature Dinners there during the F&W festival. You could look into that room. You can see the fireworks from the windows in that room.
Jiko also has a wine room, as does Flying Fish as I recall. Citricos has a quieter area toward the back with windows that look out toward the castle. Not private and would seat more than just your party, but it's quieter than the main area and a nice place to watch the fireworks.
Hope this is helpful.