First of all, thank you, Kari, for opening your heart to a horse in need. It does sound like a significant change in behavior. I have to wonder if she is in season. Some horses show significant behavior changes as they come into season. If she was severely underfed, she may have ceased cycling until she had enough body fat to cycle. Be sure to do your best to safely ignore her threatening behavior, and reward her when she is relaxed.
In order to prepare for changing bandages on that back leg, I suggest you read my answer to Laurel’s question, “How can I teach my horse to pick his feet up?” The method I suggest will help to build trust between you and your new horse.