Stephanie,
Sorry your mom has to see Walt Disney World from a wheelchair, but if there is a silver lining, it is this;
Walt Disney World really understands the strains of such a situation, and has done a great deal to make it as smooth and trouble-free as possible.
As to when to get in line, you really should get in that line around 45 minutes to an hour ahead of time, just like anyone else. The Fantasmic show has become a real draw, especially since it is not held nightly, so you could miss the show if you don’t line up early. You might check with a cast member to see if you can have her wait somewhere more convenient while part of your party waits in the standard line. They might be able to accommodate you.
You might also consider visiting the unofficial Disney fan site AllEars.net. They have some very useful articles on traveling through Walt Disney World with a variety of physical challenges.
Sorry I can’t offer more insight than that.
Have a magical vacation!