They might permit this - but there would likely be no reduction in price for the package, as you are paying for a full package and not individual courses. If you don't mind paying for an entire dinner to get Fantasmic seating, you could certainly ask.
Or you could choose Hollywood & Vine, which is a buffet, and eat only what you want.
They will box up your food if you can't eat it. I had them box up my pizza when we went, I couldn't eat a bite of it. There was no problem, and I love cold pizza It made a great snack late that evening.
1) Disney hasn't force-fed people in several months.
2) Using the F! package includes the dinner.
3) They won't stop you from leaving if you don't eat.
5) However, it sure would be a waste of money to not eat after paying.