I'm assuming you have a handicapped hang tag that you can bring? If so it should help with parking both at WBC and in the Disney lots. if you don't have one yet, you have plenty of time to do so before you go. It sure sounds like you would be eligible.
As for the buses, purely assumption here, but I believe the call ahead is so they have a bus there with the lift and additional space so a chair or
scooter would fit on board. If the walker folds and you are capable of handling the stairs then you would be in a similar situation as a parent with a stroller and for those no one needs to call ahead.