I'm not entirely certain -- but I'm fairly sure that none of the restaurants or lounges will allow you to bring your own bottle of champagne into the lounge/restaurant. Same with the parks...although you could probably bring the caviar into the parks if you wanted.
If that's the case, the best view of the MK fireworks, hands down, is going to be the dessert party at tomorrowland terrace.
If fireworks aren't a priority, then I would suggest the Grand Floridian lobby in the evening. Between the smooth jazz and the piano player, it's a great ambiance. Although, I would not open the champagne bottle in the lobby -- I would likely have it pre-opened and lightly corked before entering. Last thing you want to do is make a big noise/mess or shoot someone in the face with the cork.