She sounds like she really needs a social worker / key worker of some sort to help her into independence and access mental health support.
I would accompany her to the GP to discuss this. I suspect she would need a diagnosis first.
It sounds to me like she has not thrived living at home with you and your husband. Living away from home may well be good for her. She needs extensive ongoing support to help settle her into life living as an adult.
If you can afford it, look into DBT for her, they will teach her how to manage her emotions and the skills of thinking and behaving like a mature adult.
It may be that because of past trauma and associated mental health difficulties, she was never able to have the mental clarity to pick up and learn these skills.
Unfortunately, it does not sound like you and your husband taught her these skills, if your response to her threat of violence is an example. Gentle coaching is the way, rather than punishment.
And it definitely does sound like your husband is operating from a place of self interest. It will have to be you who is her champion and keeps an eye on her.
It is really good that you are asking questions and seeking support. Keep going and keep asking for as long as you need.
She will eventually settle down, if it is a personality disorder like Emotionally unstable personality disorder, this eventually calms down the older a person gets.
Have you tried to find support groups online or otherwise for parents of neurodivergent or “failure to launch” young or Not Otherwise Specified developmental disorders… they may have tips and techniques that may be useful to you. I think that can be a promising place to start. Maybe other Mumsnetters can point you to specific places?
Most of all, just be her true friend and give her your loving attention and affection… that may have been the root cause of all of this and what she was seeking all along. This will be very healing, as healing as any medication she might be given.
Use positive reinforcement and praise and congratulate her when she does the slightest thing right. Give positive suggestions of what you would have liked to see instead for any “bad” behaviour you see, and avoid criticism like the plague.