Whoops - You were talking about staying at MGM for Fantasmic, not going to Illuminations. Sorry - my original answer wasn't helpful after all.
We have never found park-hopping to be a big issue, so we don't tend to spend long days in any one park. Early in the morning through late evening in MGM seems like a bit much to me, but you guys may have way more stamina than we do.
If you don't mind park-hopping a bit, what about stretching out your "best meals" and making a reservation at the Brown Derby on another day for as late a lunch as possible (2:30? 3?) Then it can serve as a kind of lunch-dinner, with only a snack needed in the evening. You could then plan on an afternoon, evening at MGM, ending with Fantasmic. That would give you plenty of time at another park in the morning - either for the entire time until lunch, or just during the morning hours, with a break in the middle of the day to rest by the pool. (Mid-afternoon seems to be when the crowds are the worst and everyone's a little droopy.)
By the way, I would definitely make a reservation. We haven't found it very hard to get into the Brown Derby usually - we go to WDW at less busy times of the year - but you can always cancel a priority seating, and having one will eliminate any potential disappointment.)
Hope this is more helpful than my first attempt
