Welcome to disABILITIES.
Each resort bus has space for 2 wheelchairs (or 2
ecvs, or 1 wheelchair and one
ecv). The older buses have wheelchair lifts located at the back door of the bus. The newer buses have capability to kneel and they have ramps.
To wait for the bus with a wheelchair, your party waits at about the area the back door will be at when the bus pulls up to the stop. Sometimes there is a sign, often not. If you are at your resort, buses for all locations will be stopping at the same spot, so watch for your location and let the driver know whether you want that bus or not. We usually nod our heads and wave when the right bus comes and shake heads no if we don't want that bus. When the bus driver stops, he will open the back door and load wheelchairs before loading other passengers.
If it's a bus with a lift, you back onto the lift (ther is usually room for the wheelchair plus one person standing), the lift is raised and once you get onto the bus, the driver will tell you where to position the wheelchair. Tiedowns and a seatbelt are attached and you are ready to go. It is helpful if you know appropriate spots to attach the tiedown belts to the wheelchair. They need to be parts of the frame, not parts like antitip bars or swingaway footrests. We usually use bright colored tape to mark the appropriate spots before we go to WDW. Very easy to tell the driver he/she can attach anyplace they see hot pink tape. (If your children ride a schoolbus with a lift, everything pretty much works the same way).
For Epcot, AK and the Studio, it doesn't really matter whether you drive or take the bus. You do need a handicapped parking permit or license plate to park in the handicapped parking are.
Riding the bus to MK is always better. If you drive to MK, you will be parking near the TTC and need to take either the monorail or boat. We haven't done it for a wheile, but other posters have an mentioned that it's a bit of a hike to both of those from the parking area. As you mentioned, the monorail has a very steep ramp (have to get to a high second floor level to board and get off). The boats are OK, but if you take the bus, you skip all that. The bus lets you off right near the turnstiles so you can go right into the park.