Glad it worked out for you! I'm pretty sure even if it didn't you still would have been okay. Where I'm from, as long as you do not state that you guarantee the horse to be 100% sound you are selling it "as is".
Alot of your questions are going to depend on your local laws, your profession and how you present yourself. In Va, someone that is or presents themselves to be a horse professional has greater disclosure obligations than a lay person. However reading between the lines there seems to be a specific attempt to defraud going on. If the new buyer can document you hid know medical issues that's not just you not being a professional that's purpoosefully hiding stuff. She might have a case against you.
Buyer beware horse laws generally only cover non professional sellers who honestly don't know something and sell something as is. I don't think that is what happened here.