The handicapped parking at Epcot is closer than if you rode the buses.
At AK, the distance is about the same whether you ride the bus or use the handicapped parking, you are just going in a different direction.
At MGM, the bus stops are right in front of the handicapped parking, so depending on where in the lot you park, you may be walking about the same or about 15 "rows" of cars farther if you use the parking lot (not a great distance).
For MK, you want to take the bus. If you drive, the handicapped parking is close to the Ticket and Transportation Center (TTC). but a farther walk than if you parked in the regular lot and used the parking lot tram. Once you get to the TTC, you need to walk farther to get to the boat that takes you across the water to MK or walk up a very steep ramp to get to the monorail to go to the MK (you are basically climbing up to the station which is at 2nd floor level). At MK, you have to come down a steep ramp. If you ride the bus, the bus stops are about the same distance as walking from either the boat dock or the monorails station (so you avoid the intermediate step of going to the TTC).
I don't know about tipping (my DH handles that part when we valet park - my job is wheelchair wrangler).