We have been to Fantasmic at least 10 times and have not ever done the Dinner Package. We usually get there about one hour before time. That sounds bad, but during almost all of that time, you are actually in the theater, sitting down, waiting for the show to begin. And, there is a lot going on. They are playing Disney music, you can buy popcorn, pop, cotton candy from vendors walking around the stands and there is actually a snack bar and bathrooms inside the auditorium.
Don't worry if you see a large crowd outside waiting in line for the show 2 hours before. They start letting in people about 1 hour before the show begins; before they start letting people in, there will be a line. Once they start letting people in, the line starts moving and people who join the line after that are basically walking slowly in the whole time they are in line. We usually do not actually get into the line until it starts moving - and wheelchair users are pulled to the side so that you can be directed to the wheelchair spots.
The back row of the seating area has places for wheelchairs with seats next to many of them for another member of your party to sit next to the wheelchair. The people using wheelchairs/
ECVs can see over the people ahead of them quite well (even my DD, who is only 5 feet tall).
The 2 rows right in front of that row are also handicapped seating - for people who will not be able to go down a lot of steps, need to sit near the back for other reasons or who are with the wheelchair/
ecv parties. This is across the entire back of the amphitheater (including the dinner package area).
The Fantasmic dinner package is the section of the theater closest to the entrance (so it is on the far right as you are facing the stage). The "stage" for the show includes water and a "mountain" set that extend across the entire seating area. For part of the show, movies are shown on screen made of water mist. There is one of these screens facing each seating area (so one does face the dinner package seating area). Some of the action take place on boats that travel across the water from one side of the amphitheater to the other (so you will have pretty much the same view of the boats no matter where you sit). We have sat everywhere from dead center to the farthest section in the theater (which would be pretty much the same as the dinner package, except the opposite side). I did not think any view was that much better or worse than any other. Since the action moves across the stage, different sections will have more direct or less direct areas of all the action - there is no place that has a direct view of all the action.
The latest we have gotten there was about 10 minutes before the show began (during spring break). I would not recommend that, but we were at the park later in the day than we planned and thought we would see if there was any space. We have seen wheelchair users ushered in right before the lights go off and when we were there last in March of this year, there was no one at all in the top 5-10 rows at the farthest in section 5 minutes before the show started (so there would have been wheelchair spaces even when the show started).
You can't really know ahead of time what the demand for wheelchair spots is, but I would expect to get in if you are there an hour before.
The CMs at the Information area can give an estimate of probable crowd levels and what time you would need to get there. I have heard of people coming later and being turned away because all the wheelchair spots are filled.
If you do decide to use the dinner package, let them know you will have an ECV/wheelchair user in your party so that they save him one of the wheelchair seats in the dinner package area. I don't think there would be any advantage to having a wheelchair over an ECV. On our last trip, my DD was using her wheelchair and her grandfather was sitting right next to her using an ECV. The way out is long, with sloping walkways and almost no benches, so I would not suggest leaving the wheelchair (also, once you are in the amphitheater, you may not be able to get back in if you leave).
) ? Also, how is the seating for wheelchairs/ECV's there, is the visibility good? If the visibility is poor, we're considering getting the dining package regardless, renting a wheelchair to get him to the restaurant instead of bringing the ECV, getting him to the dinner seating at Fantasmic and then I'll run and return the wheelchair if the dinner seating is far above par from the handicapped seating. He says he'd be fine waiting until the crowds leave and then hitting a LOT of benches on the way back toward the bus, but I think he's nuts. 
Thoughts, opinions?
