If my 78-yo mom was in a wheelchair, and waiting at the back door position for the next bus, I would not leave her alone there while I went in the regular line. I am sure the same sentiment would be expressed by parents of a child in a wheelchair. Leave their child alone waiting while they get in the regular line?
I think Disney's rules for bus loading are crystal clear, and there is no need for second-guessing. The wheelchair or
ecv, plus up to 5 others, wait at the back door position so the bus driver can see the situation as the bus approaches the stop. The bus driver will keep the front door closed, go to the back door, open it, and load those people first if there is room, or deny loading if there is no room or if there is equipment malfunction. The place for asking the bus driver if you could fold up your wheelchair and still board is there at the back door.
Making a show of being in the regular bus line, to the point of leaving your disabled loved one waiting alone in the wheelchair bus line, is unreasonable and heartless. The Disney approach is sensible.