We normally allot 1 to 1 1/2 hours for table service meals. I would think that you would have time for your meal and time to see Illuminations. Like a previous poster said, you might not have the best "seat in the house" for Illuminations but you should finish in time to see the show.
We usually show up about 15 to 20 minutes prior to our ADRs. Last June, our ADR wasn't until 8:20 p.m. but it started to sprinkle rain and we headed over about 7:15 p.m. They told us we had to wait which we already knew. We were just trying to get there before it started to rain harder. We were very surprised when, just about 10 minutes later, they called our name and we were seated. They said it happens sometimes.
If you are near LeCellier earlier, you might want to stop by and ask if they could fit you in earlier. There may be a cancellation or no show.
Have a great trip and eat lots of good food! Lisa