I'm sorry to hear about your guys injuries. I'm not sure about the negligence, I think that it would depend on the laws of your state but it probably falls under some type of protection law for a boarding facility.
When my horse severed her tendons, she was given about a 70% chance of being sound. Her injury was very severe and required emergency surgery so it already sounds like yours is better off then mine was. She healed beautifully. She would not have a scar at all accept she got a fungus on the front part of her leg from being wrapped for that many months. Now she has white speckles on the front of that leg. It is also fatter then the other legs (due to scar tissue), but not by much. She is a bit off. She is not in pain, sound for riding but there is a mechanical lameness. She trail rides, walks, trots, canters, and even jumps on occasion. There is hope. She does not have stringhalt at all.
There was another horse that was injured on the same weekend as her, they opted not to have the surgury, there was an infection and the recovery time was a lot longer than mine. That horse does have stringhalt, BAD at the walk, not noticeable at the trot or canter. The girl that owns it still shows and places. At the trot, her horse has no lameness where mine does. At the walk, mine is just about perfect, hers has the stringhalt.
"Be a best friend, tell the truth, and overuse I love you
Go to work, do your best, don't outsmart your common sense
Never let your prayin knees get lazy
And love like crazy"