The only sit down place to eat in Epcot to watch IllumiNations is Rose and Crown. Try to check in earlier than your reservations and request an outside table by the water (there are also outside tables on the side of R&C but you probably cannot see IllumiNations from there). You are not guaranteed a seat but there is a good chance you will get one if you wait a bit. I believe if you eat inside the Rose and Crown there is a separate area you can go outside and watch. The past three times we have eaten there we have had 7:30 reservations and we are usually having coffee and desert while IllumiNations is going on. There are umbrellas outside but they close these for the show.
To the left of R&C is a garden area with tables and chairs that you can get counter service food and sit and watch the show also but this area fills up fast.
The only other place you can eat and watch the show is the counter service in Mexico - but you need to get there early as tables go quickly. You can get counter service and watch IllumiNations.
Linda