Maybe doing it from upstairs at the railroad at the front of the park? That would have the castle and fireworks in full view while not taking away from the more important subjects in the photo and would be easier to coordinate. Would also be easier to skip off to a celebratory drink at a monorail resort (if she says yes).
I know the train doesn't run during the fireworks, but isn't it still open upstairs? I don't really remember.