I would try to change it to 7:30. If you show up at that time or a few minutes early, you'll probably be seated between 7:45 and 8:00 and you'll be eating or enjoying dessert while you watch Illuminations. We always ask for a 7:30 seating and have never gone wrong with it. I suppose you could stretch your meal out from 7:00, but you'd really have to take your time ordering and eating (maybe have a drink before you order) in order to still be there at 9:00. There comes a time at Rose and Crown where, even if you have time to kill, they all know you're not leaving before the show is over.
Good luck and enjoy. I love Rose and Crown!