I wanted to add another vote for the consideration of euthanasia, and also an empathetic hug to OP. It’s terribly hard to be in a position with such a horse, and I’m in it with one right now.
OP - if you have Facebook or Patreon, check out the necropsies Beck Nairns is doing on horses. She has discovered MANY skeletal abnormalities that aren’t normally seen by veterinarians, but cause the horses tremendous discomfort and pain (as the horse looks happy in pasture or even in work). Horses are stoic and won’t always let you know that they are hurting. I would absolutely do spinal and neck x-rays at minimum on this horse, or donate her to a university if you can find one that is doing similar work as Becks Nairns. Pain doesn’t have to be constant either, we all know that. For those that deal with chronic pain, some days are 6’s and some days are 10’s. Nerve pain is a whole other can of worms.
I too know someone who had a horse rear and flip over on them, as a teen. She had her pelvis crushed and was in a wheelchair for a very long time. I don’t think she ever returned to riding, only coaching. And not that it matters, this horse was a perfect, all-around, well-seasoned, safe, safe, safe show horse. She rode over a ground wasp nest and the consequences were terrible.
It’s not worth risking your life on a horse that was deemed dangerous by many people around you, seeing the situation in-person.