Dinner seatings start around 4.30 I think, but check the MK opening times before you make your PS to see if you can coincide it with the fireworks. 'Ohanas has big panoramic windows (so when you check it, ask if you can be seated by one) and they dim the lights and pipe in the music. Out of season, if they have fireworks, they can be as early as 6pm.
As for the jet-lag, I doubt it will be too much of a problem. I always find it's far easier to stay awake later when I've gone out there because I'm excited, rather than losing the 5 hours coming home!