The wheechair seating is the back row of the auditorium. There are spots for wheelchairs and then a few seats next to them. The row ahead of the wheelchairs are reserved for the other people in the party.
I don't think they have specific wheelchair seats reserved for the Fantasmic dinner package; you just would end up in the regualr wheelchair seats. Make sure to let them know when you make your PS that you will have a wheelchair so that they save you a wheelchair spot.
We have sat (in the back row) of every section from the far right, dead center to the far left. My personal opinion is that there are no really bad seats. The stage is a semicirle with a "mountain" set in the middle. There is a "river that goes around the stage between the stage and the audience. The seating area is a semicircle that follows the stage. During the show, some of the action takes place on the stage, some on boats in the water and some on large water screens. The action moves to all parts of the stage, so there is not one area where you will see absolurely everything. Everyone gets an equal chance to see the boats as they come onstage on one side and travel around the stage to go off stage on the opposite side. There are 3 water screens, so each area of the auditorium gets an equal chance to see those.