I read the entire thread and watched all the videos.
IMO…
My first impression was a pain issue… something seemed NQR in the way the horse uses his hind end.
The at liberty video caused me to consider a behavioral issue.
Oldest video of horse shows the horse was leaner and had less muscle at that time. Certain sellers have been known to turn out “hot” “difficult” horses 24/7 and lower their feed while keeping them in work. It can have the effect of lowering the horses energy levels, and can result in a horse that appears to have a much mellower temperament. Buyer gets horse home, puts the weight back on the horse, and then they have a firecracker on their hands. It can happen.
OP, I hope you see results from the ulcer treatment.
I’m very reminded of a number of TB’s I’ve worked with over the years who were just “hot and difficult horses”. Some horses that are unknowingly worked with pain issues, may essentially develop a form of “stress disorder” (think Post Traumatic Stress Disorder in Horses). The horse learns to associate work with pain. If the pain issue is later resolved, the stress disorder can remain. Some circles may refer to this as a soured horse. Retraining a horse with such a stress disorder, first requires resolution of the stress disorder. If the stress disorder has become deeply imprinted, the horse may never fully recover behaviorally.
The recent vet diagnosis of lameness in the left hind may or may not be the overall cause for this horses behavior.
I hope this horse’s issues are resolved and he may lead a happy life.
But in my career there have been certain horses who never became trustworthy mounts for anyone. The horse I’m thinking of had a reputation of injuring riders, professional riders. In the end a decision was made to put the horse down. The decision was based on preventing anyone in the future from ever becoming injured or worse. If a “dangerous horse” is retired as a pasture pet, and out of the hands of professionals. There will always be an element of uncertainty that years later someone may try to ride the horse again and get hurt. In the end, I personally will always put the welfare of people above that of the horse.
Hope you never have to make that decision OP. Let “your vets” guide you if it comes to it. You are lucky to work with professionals who may provide guidance for you.