We have been to Fantasmic even in very busy times (like Spring break) and have not had a problem getting in with a wheelchair.
You will see people start lining up in front of the entrance to Fantasmic pretty early (like starting an 90-120 minutes before the show). It's NOT necessary to join that line up.
About 1 hour before the show actually starts, the CMs open the gates and let people begin coming into the amphitheater area. The 'mob' that had been waiting outside will start going in as one large, slowly moving mob of people. Within a few minutes, they will all be in the amphitheater area and it will be much easier for the next guests to get in. So, don't try to go in until you can see the line moving. People using wheelchairs and
ECVs should stay to the right (unless you are directed by a CM to go somewhere else). Before you get to the actual seating area, you will be met by a CM and escorted to a spot.
The seating for people traveling with wheelchairs/ECVs and other special needs who want to sit in the back is in the last row of the amphitheater. There are a lot of
ECV/wheelchair spaces in the last row, with one companion seat next to each. The rest of your party will sit in the row(s) directly ahead of where the ECV/wheelchair is parked. The seating is the same for people using ECVs and wheelchairs whether or not you are using a Dinner Package (i.e. wheelchair and ECV users park in the back row).
One other suggestion - when the show is over, take your time leaving. The majority of the audience will try to get out quickly. This means it's very crowded and congested. We always wait until the crowd has thinned out considerably before we even try to move. We also usually do not go out using the path that mosts guests are taking (all the way across the top; the same way we came in). We usually follow the path that the guests who used the dinner package used to get in - go all the way across the amphitheater toward the exit. Once you get to the end of the row of seats, look for a ramp that goes all the way down to the front of the amphitheater and go down that toward the 'stage'. You will find a path on your right that will take you out of the area and toward the front of the park. YOU WILL be in a backstage area that goes by the back of the Beauty and the Beast theater; that's where you are supposed to be. Just keep following the path around and it will take you back into the front of the park area.
This thread shows some maps made by a posted called Robo that show what I described.