When there are two shows, the second show uses the alternative exit for wheelchair entry. We've done that. You wait until everyone has left the theatre. Then before they let anyone in using the main gate, they let the wheelchair parties in. We waited 20 minutes or so, but wheelchair parties joined the line right up until they took us in. (Granted the earlier you were in line, the closer you were to the exit when leaving.) We were seated prior to the main gate being opened. So we were never in a crowd. Leaving is another story unless you are ahead of the crowd, don't have to stop for a stroller and can "move out".

But you can always wait until most of the crowd is out before exiting. Maybe with a GAC you can use that entrance if you go at a time with two shows on one night. Worth asking.
I don't know what their criteria for using that area for entrance is, but I have not personally seen them use it, although you are not the first person to post you used it.
Our last time at Fantasmic was in October 2010 on a night where there were 2 shows and they were not using it that night.
We arrived about 50 to 55 minutes before the first show was scheduled to begin. At that point, there was a short line to walk in and they were announcing Standing Room only. DD is in a wheelchair and I also showed her GAC to several CMs going in, they said GACs were not being used and just to keep going. I asked if GACs would be used for the second show and whether there would be a place to wait with wheelchairs for the second show rather than the large line.
I was told there would not be, so we decided to just keep going in for the second show. We did not know how many wheelchair spots there would fopr that show and decided if we got there and all were gone, we would go to the second show.
We ended up with the next to last wheelchair spot for someone who needed to remain in their wheelchair.
We were directed to the farthest seats on the left side of the stage (the very end seats). After the show, we waited to leave the show area until most guests had left. The path leaving crossed in front of the rope that entering guests were waiting behind. We saw several
ECVs almost at the front and several guests with wheelchais toward the front of the crowd of people, so they had been among the first to get in line and were not pulled out.
As we went past the entrance to the Fantasmic theater, we saw that half the street was roped off to form a waiting line, which stretched almost all the way to the Vilians shop. We saw at least 12 guests in wheelchairs and EVCs mixed in the line with the other guests from the Fantasmic entry to the end of the line.
This was our experience in May. There were thankfully two shows (and the second show is typically less crowded) so we rested up a bit that afternoon so he wouldn't be exhausted and saw the later show. There will be an initial dump into the stadium when they first open the ropes. After that it shouldn't be that large of a crowd coming in. So we waited until they opened the ropes and walked up without much fuss.
Once arriving I asked, showed my GAC as well, if we could be permitted to sit in the back row away from the water sprays and with easy access out should DS7 become overwhelmed by the show. We were obliged and when we sat down a bit of pixie dust happened in that the CM who was working in that section worked with ASD children in her home state and spoke with DS before the show about exactly what to expect, heck she even knew the medications he was on when we started to talk about them.
HTH!
If there are 2 shows, the second is typically less crowded.
For both shows, they open up the ropes as much as an hour ahead of the show to get people seated. While guests are waiting for the ropes to open, the lines are long.
Once the ropes have opened and guests are getting seated, the line goes down very quickly.
The very last row of the theater seating is set up for guests with wheelchairs, ECVs and other mobility devices. There are 2 parking spots, then a small bench for one companion to sit next to each wheelchair. The 2 rows ahead of that row are for guests traveling with those using mobility devices or those guests with GACs who need to sit near the back.
If your child is using a stroller as a wheelchair, it could be parked in the wheelchair spots, but those of us whose family member can't move appreciate guests moving and sitting on a bench if they are able. You can take th ability device to the seats and then a CM will move it.
That way, a child who needs the security of a stroller could stay in it for as long as possible.