This was in the other thread, right? OP clarified that they were actually saying their horse does NOT do that. That would 100% be a neuro giveaway IMO.
What tests? What mistakes? This is so vague. I agree that this vet seems to be throwing shit at a wall to see what sticks. There is a sports med vet in my area that put me through the same ringer. Injection after injection without actually finding the cause of the problem.
You mentioned many times in this thread that the saddle fits badly. This will cause atrophied muscle and be hard to fix in the long run. The saddle fit, along with the narrowing spine columns, could definitely be causing the bucking. At the end of the day, regardless of the issue, this horse should not be ridden right now, at all. You will likely have to do extensive work in hand to fix the damage done by the saddle/rider.
Definitely get a second opinion on this horse. I am NOT discouraging getting a neuro vet to look at this horse. You absolutely should, because it is an important thing to rule out. I wasnât trying to be difficult with my comment, I was genuinely curious because this didnât present as obviously neuro to me.