There are limited spots for people traveling with wheelchairs and
ECVs, but the wheelchair seating is the whole top row of the amphitheater, all the way across. Whether or not it fills up is not dependent on how busy it is, but on how many people are treveling with wheelchairs and ECVs.
We have gone at least once a year since Fantasmic opened and have never used the dinner package. We have always gotten a wheelchair spot for DD. We generally get there about 45 minutes to an hour before the show begins. We have gotten there as late as 10 minutes before the show started and still gotten a spot, but have also seen all the spots filled by 30 minutes before the show begins.
You may see people start lining up 2 hours or more before the show. When you see that, it looks like there will be a long line to wait in and people start to worry about the wait. The CMs actually open the gates about 1 to 1.5 hours before the show starts. Once the gate has opened, there will no longer be a line of people; instead of standing in line waiting, it becomes a slowly moving group of people all the way into the amphitheater. People with wheelchairs, ECVs or special needs are separated out at the point of the stroller parking area and you will be shown to a spot by a CM.
The advantage to the dining package is that you can show up 30 minutes before the show, rather than 45 minutes to an hour. Even though an hour seems like a long time, you will be in seats and there is a lot going on to look at - people moving around, the crowd doing the "Wave", etc, so the time goes much faster than it seems like it would. If you have a person who does not want to sit, you can have most of your party sit while 1 or 2 people leave the seating area and go somewhere quieter. There is a set of restrooms out near the stroller drop off point where you could wait out of the hustle and bustle of the crowd. As long as you know where the rest of your party is sitting and you don't leave the attraction area, you will be OK.