There are a lot of passionate arguments on your post that are only tangentially related to you and your specific horse. Many people are picking sides and getting personal. You’re in a sad situation and don’t deserve a pile on. You happened to purchase your horse from someone who inspires these sorts of arguments. A lot of these replies aren’t even about you…things went left.
Leaving the seller out of this, that lunging video is objectively not fair. Leaning back for leverage the way the trainer is doing is a dirty move. Granted, she didn’t know his medical diagnosis at that point -but even if his back were pristine, that’s an ugly thing to do. Maybe it was initially an instinctive reaction, but she continued he session. She doubled down?
Those methods might get her discipline from a different type of horse, but they just aren’t going to fly with a sensitive, subtly communicative, extra good boy of a thoroughbred. Even if he were pain free.
It’s also not fair to introduce the horse to lunging that way, especially in a big open ring/arena. A horse does not magically understand the goal there. He’s not given any hints about what to do or any reward for doing the right thing here…just pressure or more pressure. He doesn’t come equipped with knowledge about that whip pushing him out and all the ropes yanking on his face. That’s not teaching or training.
You’re horse is trying SO HARD to be kind despite his pain, and his efforts (from his perspective) are being punished.
All you have at this point is his beautiful, eager to please temperament. That’s a treasure. Treating him that way is putting him in a position to be forced to be more…demonstrative. It is putting his temperament at risk.
I understand that you must be exhausted. This trainer was your soft place to land, so the last thing you want to hear in the midst of the avalanche of criticism in this thread is that the trainer isn’t the right fit. Sorry. Get this horse into the right hands for rehab. You can revisit his current trainer later on if you want.
Rehabbing KS is specific work. Anyone kind of qualified to do that would have been waiving KS on those videos. They’d certainly recognize it in person before hopping on. Ideally, a trainer has the humility to say “I’m not the best trainer for this horse right now. I’ll help you find the person who is and bring him or her in, or send you there.”
I appreciate that you’ve been willing to share here. It really is a cautionary tale that could save some people.