I agree with what everyone said about getting another vet to diagnose your horse. I also understand having to put that on hold due to your health. It sounds like you are doing everything you can to ensure this is one well loved ans healthy horse, that is wonderful! I have one other suggestion, and I'm not sure how you feel about natural treatments, but there is a company that is in Idaho that does herbal supplements. They were just a few miles from where I used to live and I've taken a few horses over there for help, Mickey used to do checks for reactive points on their meridians and then based on that recommend diet and herbal blends to support the horses nutritional and system needs. I like to tackle problems both with traditional vets and with herbs. The company is Silver Lining Herbs, family owned, super helpful people, you can call them and even without a diagnosis from a vet they will review the symptoms and make suggestions on herbs. This isn't a magic cure, but I have seen them really make a difference. I've used them for horses and dogs and shared their info with my friends who use them now too. It isn't cheap, about $1/day per supplement, but it is a low risk option to try in my opinion. I've treated cancer on my mares eyeball with them, low immune systems, West Nile, systemic infections, navicular, old stiff joints, etc.
They're worth some research if you're open to it. I've seen enough horses, dogs, and people positively helped by the herbs to be a firm believer in what they can do! Oh, and just last year our mare degloved her shoulder, tore through some muscles too, about 12" of stitches internally ans 18" externally. We put her on Inf-X and Immune Support, we used the antibiotics as well from the vet, she healed beautifully. She's a broodmare, and can still run just fine and has full range of motion.