I think it'll be much simpler to get dessert at one of the CS places, such as the Patisserie or Kringla, funnel cake at the American Pavillion, crepe at the crepe stand... and find a spot near the railing. I know there are benches in the Norway/China area.
There are good viewing areas all around the lagoon. Just choose one where you can see the center of the lagoon and isn't obstructed by the small islands or overhanging tree branches. If it's windy avoid being downwind.
While having dessert at R&C sounds like a good idea, there are a few potential drawbacks to the idea.
--First of all, you'll need a reservation. You won't be able to walk up and be seated just before IllumiNations.
--Most people who have reservations starting around 7:30 probably plan on staying through IllumiNations. It would be hard to have a reservation that early and stretch dessert and coffee to 90 minutes and not many tables, especially on the patio, open up after that.
--Only the tables on the patio have seated views of IllumiNations. You could request a table on the patio, but the CMs at the podium might tell you that you'll have to take the first available table, especially if you're there when it's busy - such as summertime, F&W, weekends, or holiday seasons.
--There is an area adjacent to the patio where people dining inside R&C can go out and see IllumiNations. We've done this and then gone back inside to finish our dinner.
