we just returned 6/18 from WDW. I was temp in a WC for a broken foot.
EVERY bus driver, without exception, very enthusiastically greeted me at the back door of the bus, operated the lift, and expertly tied in the chair. They are very good at what they do, and they do it in a very friendly and professional way. You should have no trouble.
Be advised, though, that the drivers are not permitted (not supposed to) handle your chair, for liability, so MIL will need to be able to drive the chair up the lift ramp and into the bus and get it positioned where the tie ins-are.
I have never stayed at All Star, but I hear the busses get crowded. If the wheelchair seating area is already occupied when the bus arrives for you, the people will have to vacate those seats for your MIL's chair. It is the law. Insist if you must, although the drivers will be your advocate.
k