Botulism would be from a dead rodent etc in the feed, which happens in hay.
Allergic reaction ? Poison ? Chemicals?
If people have already harassed you and you had to sell a horse ? Because of your blog.. I would stop blogging, I would have moved the horse .
You are in Nevada and no Vet will go to the barn ? Something sounds fishy to me. There is something 'off ' here. Either you are exaggerating or leaving out lots of information.