Oh, my gosh. I just read through this whole post, and I can't believe all these stories!! I would absolutely, without a doubt, move my horses out of there. And I wouldn't worry about giving any 30 days notice either!
I would also report the place to the ASPCA. That barn manager sounds like Hitler and I can't believe everyone is putting up with it. Is it hard to find boarding places where you are? It sounds like it must be for people to put up with that kind of treatment. And to pay that kind of money??? That's a lot of money, in my opinion, for the kind of treatment you and the horses are getting.
I've never heard of a barn manager who would get involved in your horse's vet call. That's insane. And to put other horses in your horses stall is bad enough, but especially when horses are sick??? The new horse who came in sick should never have been allowed in. And it most certainly should be isolated at the very least.
Man, I'm getting myself worked up here and I'm not even the one who is dealing with this. Grrrr.