Do you have your reservations made already? Are they in the park?
If you don't have your reservations made already, then you're probably at a "Take what you can get" point already. July 4th is one of the three busiest days of the year at the MK. It consistently closes for the day, because the park is at capacity. There aren't a lot of TS choices at the MK, so I imagine that the reservations fill up quickly.
I would not leave the park once you're in the park, and you'll need to be in the park early to make sure that you can get into it for the day. So if you have a dinner reservation outside of the park, my suggestion is to cancel it.
If you do have a 5:30 reservation inside of the park, you should be ok. I definitely wouldn't eat much later. Fireworks are at 9:00 that night, and people will start lining up early. A 5:30 dinner will get you out of the restaurant no later than 7:00, and should give you some time to line up.
If you don't have any dinner reservations yet, I'd make a LUNCH reservation, and then start camping out for the fireworks around dinner time. You can have one member of your party run for food (hot dogs or something similar) as you wait and they can bring the food back to you to eat while you wait.
Sorry this is so long, I didn't know enough about your situation to give just one answer, and didn't want to reply with "it depends", because that wouldn't have been helpful at all.
