As another perspective of one who was mobility limited, my husband found the Viewliner H unit worked adequately for him. (We never traveled in a Superliner H unit.) We used a lightweight folding transport chair (basically a wheelchair without the large wheels, so it must be pushed), as he could not walk for much distance. He did have some difficulty walking the length of the car to the dining car, but he could do it. (Airborne spirit!!) He also had balance issues but the narrowness of the corridor helps with that. He would board in his chair using the lift, but could then sit on the sofa. I think he'd have personally preferred walking further to the diner over riding facing backward. He could manage the H unit's restroom alone if I helped him up from the sofa.
The last few trips, he spent mostly in his chair, and we had our meals in the unit.
I think Amtrak's policy on selling the H unit is a reasonable one. My husband would have never expected nor consented to kicking out a ticketed passenger to offer us a last minute accommodation. We've have figured something else out.
As Jim often says, YMMV.
Edit to add: We have seen all the handicapped parking spaces taken, but only at doctors' offices or other healthcare facilities. I don't doubt it happens sometimes at other places, though.