I haven’t read past halfway down page one, and I haven’t seen a video. Just wanted to pop in with my 2¢ though, as I lived a similar nightmare.
I was looking for a project horse and picked up one with a bad history of very little riding, poor start in life, chronic bucker, had flipped, aggressive. Looked sound as a dollar at all 3 gates and seemed to have heart in there someplace, so I took on the challenge.
Had my vet go over him the day after he arrived and we all agreed he was fine.
Horse was a confirmed balker too, so assuming he didn’t know squat we started from scratch.
A few months into training, horse was obedient on the ground, balker ONLY WITH A RIDER UP. Western saddle, fit quite well. Aggression only got me broncing from a standstill, so I did the patience/wait him out thing. In the end that worked, and after about a year I had a horse that would move off my leg and was more or less obedient, but the tendency to buck and get out from under the rider, was always there. Extreme naughty behavior would surface once in a while. I never felt safe or relaxed on this horse, and only once in a while would horse feel really happy to be ridden.
Fast forward 3 years now. I’ve been having a strong nagging NQR feeling about the horse all along, have had my vet go over him 2 more times, had two chiropractors and acupuncturists work on him, nobody can find a thing wrong with him. He is more or less obedient, more or less well behaved, but lessons are short lived, I feel like I have to repeat myself more than needed especially with a horse that picks up everything on the ground so quickly. I’m convinced there is something wrong, but horse appears sound as a dollar, athletic, even, flexible, supple. Just isn’t a happy horse with a rider on its back… always a bit sneaky, always ready to balk, taking any opportunity to get the rider off its back. I feel like I’m riding a ticking time bomb and I can’t shake the feeling.
I finally decide it has to be me - over the years, finding myself being thankful no one got hurt over and over and over, the horse had chipped away my confidence - and hire a pro to ride the horse. She gets the horse to go, and stays on through his antics, but the horse is furious. After 3 days of riding, otherwise pocket-pony horse refuses to be caught to go work. This is the last straw for me, and I ask my vet out to do a 4th evaluation on this horse. I beg her to xray his back, he had flipped in his first year under saddle with his former owner. Xray reveals broken withers.
He.had.been.in.pain.all.that.time.
All the bucking, all the carrying on, with me, with the former owner. YEARS of him protesting as loud as he could, refusing to walk forward, how much more obvious could he have made things? Nobody caught it. Vets. DVM Chiropractors. Acupuncturists. Trainers. Observers. No one saw a horse in pain. He never sunk his back away, never said “ouch” as we recognize it. He would just get pissed and lash out, that was his way.
He is now a driving horse and sweet as pie, happy as a clam, thrilled with his new job.
Just need to share this, YEARS of my life could have been saved if only I had followed my hunch early on and had him xrayed instead of listening to all the people around me who said “he’s just a jerk. He learned to be bad”, etc.
What really broke my heart was when I looked back on how far we’d come and realized I’d trained him to accept the pain.
I’d trained him to deal with it, and he did it best he could until he just couldn’t take it any more. 
Good luck