OP, I'm really sorry you are so angry about this.
There's a LOT of folks here who are either in chairs/mobility devices full time, or who have a loved one who is, so a LOT of us deal with the bus drivers - and getting on/off the buses - just like you and your hubby do.
In fact, it's probably one of the biggest topics of concern for Guests who find that they have to use a mobility device at WDW on their next trip. There have been *pages* written on the subject here - including telling folks to make sure they are in the white box on the ground (or in line behind the person in the box) and signaling the driver as the bus arrives and before passengers get on.
I'm just here to say this: Probably 99 times out of 100 when I ride a bus at WDW, I have zero problems with the driver. That one time is typically at the end of the day, when the driver is in a multi-stop Resort and forgets to let us off, which usually results in some hollering from the middle of the bus before he pulls away from the stop!
We have had drivers - and this happens to us more at the Theme Parks during the middle of the day, or at Disney Springs - who will have already loaded ambulatory passengers, and then we roll up before they depart, and so they shuffle around some folks on the inside, I roll on, and away we go. That's happened more times than I can remember.
I'm not defending the driver - because I wasn't there to see for myself what happened. But I have seen drivers load mobility devices after other Guests when they have arrived after regular boarding began. And I have seen drivers do this not once, but twice on one bus when people with wheels arrived at two different times before the bus was scheduled to leave.
And I have also seen drivers tell folks who roll up after the bus is loaded that they will have to wait for the next bus. I've been that person! LOL I don't really mind, because these days we can just call a Minnie Van, or... in 20 minutes (or less) another bus will be along to the same destination. We always double our transportation time when planning bus transport at WDW, simply because we know that if more than 2 mobility device users need to go to the same place, someone is going to be left on the curb waiting.
Anyway, all of this is to say: What you experienced was not that far outside the norm, especially during busy season at WDW. I know it was unexpected for you, but it does happen, and more often than you have experienced in the past.
I am at WDW an average of 3 times per year as well - and we tend to stay between 7 & 10 days at a time, so I spend a *lot* of time dealing with Disney bus drivers. I doubt very seriously that the young man acted with any ill intent or malice in his heart specifically towards y'all. I don't know why it happened, but it's over; please don't let it be the defining memory of your vacation - I hope the rest of your time there is as Magical as it can be!