It actually is an easy job and is nothing like the common perception. 99.999999% of the people are nice. And most, like yourself, do say thank you, and I think most appreciate the service. But it wouldn't matter if that wasn't the case, drivers get paid to do a job. Thanks for kissing up though.
Now for your other points, I agree with you fully, however Disney managers, not the lowly drivers, decided what type of bus to use and the sitting to standing ratio on those buses. They also established the loading and boarding procedures that you experienced. And the resorts decide what level of service they want to contract WDW Transportation to provide.
As for a rough ride, it could be due to the no frills, lowest bidder buses, and possibly a driver that is not all that they could be.
The scooter abuse situation is just as annoying to the driver as it is to you.
I have ridden as guest on a packed bus a number of times, and like you, wasn't all that impressed with the ride some of the times.
All I can say is that "it is what it is" and most drivers, not all, try to make the best of what they have to work with.
Don't know if that's a satisfactory response, but that's all I got.