On our last trip in December, we had a late ADR at Le Cellier (not an EMH night), but the hostess made it very clear that if the entire party was not present when our name was called, we would not be seated and we would lose our reservation. I heard them telling another group the same thing - that if they left the waiting area to go watch IllumiNations, then they would lose their reservation if their name was called and they were not there. If you want to eat at Le Cellier, I would not risk the reservation by having part of the party not present.
<Edit: Added Info>With regards to the seating time. It varies. Sometimes the resturant is on schedule and you'll be seated at or close to your ADR. Perhaps even before if you show up early, but it's just as likely that things will be behind schedule and you won't be seated for a while. We had one dinner reservation (8:50 pm) and we were seated at 8:50. We had another for 9:05 and weren't seated until 9:35. So it really varies.
Also, as PP's have indicated, even in early December when the crowds were low, all of the Soarin' FP were gone by mid- to late-afternoon. There is very little chance that there would be FP at 9:00 pm, even for EMH. (Actually, do they issue FP for EMH hours? Two years ago the last available times coincided with park closing.)