Hi there, I would say 7.30 or 7.15 at a push. Having said that, if you come along and are having 3 courses, 7pm would work. As long as you are eating/drinking and leave a good tip servers don't seem to mind!! It also depends how busy the restaurant is - it could take 45 mins for your dinner to arrive!
You can't guarantee seating on the patio area, but if you are sat inside, don't worry - I believe they still have a roped off area for guests dining inside to see the fireworks.
Also, if you get to the Beer garden next to the R&C by about 8/8,15, you should be able to get a great seat there, with an almost identical view of Illuminations. You can get take-away drinks from the Chip Shop or the R&C pub, and can grab fish and chips to eat there too!
Have a great vacation!