I'm sorry you had such a horrible experience, and sorry for your loss.
I must say, however, that I have never seen a miniature horse that needed sedation to have it's teeth done if the dentist ..... and the owner .... know how to handle a horse.
I have worked with 2 different well-known equine dentists that specialize in miniature horse dentistry. Even the rankest, rammiest, most rambunctious horse didn't need any sedation and in fact, if handled correctly, will stand quietly. My 24 year old stallion hadn't had his teeth worked on at all during the first 16 years of his life and being an old prairie boy, can be a handful when he feels pinned in. He stood quietly and almost went to sleep during the process and he needed a lot of work the first time!!!!!
You unfortunately ended up with a dentist who doesn't know the first thing about how a horse should be constrained while having their teeth worked on.
Which brings up a point ... if anyone is considering working with a dentist for their miniatures, one of the questions they should ask BEFORE letting them work on their horses is whether or not they use sedation when working on them. If they say "yes" ....... find a different dentist.