I don't understand "testing" bute/banamine to "see" how a horse reacts. I don't know, maybe that stuff is like aspirin to a horse (or maybe not?)
Regarding blindness: I've got blind.
I learned he was blind AFTER lightning struck our tree, wiped out most of our electric, and somewhere along the line Nicky was struck in the eye with either a cedar tree shard or a piece of melted fence charger. I only learned while he was being treated for that injury that he had cataracts and was already significantly vision-impaired. Now, I know. But if you saw him puttering around, you probably wouldn't know he was blind. In retrospect, some of his behavior makes sense, but they really are pretty good relying on their other senses. Just saying....
Regarding desensitizing: We did a lot of "desensitizing" simply because we didn't know any better. For example, tractor had to go in for stall clean-ups. In it went. Other than ensuring they stayed out of the way, we didn't know any better. The husband used to start up the Harley (which was really loud with some sort of straight pipes on it) a couple feet away from the fence and Nicky's head; Nicky wouldn't even flick an ear. None of them do--for noisy stuff we do. In fact, some noisy things mean 'food" to them, like for instance a weed whacker.
I'm not where I want to be with our mare (the other two, I think I'm doing ok.) but with the mare I'm starting to wonder whether she's being a douche just to get the attention.