OP this might seem an odd example but she reminds me of someone I dealt with professionally in a previous life.
I was this person's line manager, and I suspect she must have had some sort of personality disorder but without going into detail imagine how this girl might act in the workplace and you'll have some idea. It was like a totally different person to the one who had been at interview came to work. It became immediately clear this person could not be exposed to our clients, based on her behaviour with people she did interact with, and so the decision was made (higher up than me) to terminate her contract while under probation, literally in the first few weeks, based on interactions she'd had with senior management.
We had got legal advise, everything was done by the book, she had no legal recourse, but we had a public contract at that time and she used whistleblowing legislation to claim she had uncovered our misuse of public funds and was dismissed in retaliation. It was a complete fiction, but obviously cost a lot of money and reputational damage to disprove. Reading her statements, and the few times I had to meet her as part of the process, really highlighted to me how disturbed and manipulative she was. She fixated on me as the orchestrator of her downfall.
We had documentary proof that showed I wasn't the person who had led her dismissal, but she still argued that I had manipulated everyone out of dislike for her and fear she would uncover my (imaginary) wrongdoing. In reality, I had been nice to her, if increasingly concerned and alarmed, and hadn't even got to the point of raising my concerns with management before they came to me. I really think she believed at least some of her own lies, not at the beginning but that she came to. I worked along her for only a few weeks but I still, many years later, feel she is someone who would try and do wrong to me if she ever had the opportunity - like I have an enemy out there who wishes me ill.
That is exactly your scenario with this girl. People like this do not grow out of it. She will not become more mature, she is very unlikely to fundamentally change. Something is wrong with her. And she will use your son to feel better, and she will continue her vendetta against you because when something is wrong with you on a fundamental level it is a lot easier to have a clear 'villain' you can blame it all on, especially when you are being enabled by people around you.
A v different scenario I know but I cannot tell you how relieved I was that we had documented everything we needed to, and I think you need to do the same. Imagine you find yourself, worst case scenario, in a few years supporting your son in fighting for custody of your grandchild, or defending your son against allegations from her. You need the restraining order - you need proof your family took threats from her seriously. You need to be accessing domestic violence support from the police as much as you can, and making records of what has been told to the school, what they say, basically as much external support and proof you can get.
What is happening will be twisted and distorted by her, and while its impossible to know the best way to support your son - and I think what you're trying now probably is the right choice - you can't allow yourself to become unclear about what is going on, or her motivation. She is an abuser, and you need to keep that to the front of your mind at all times.