I've something a bit similar with an abusive family member, social services and police involvement due to false allegations.
A police officer told me that cases like this are hard for them to deal with, as the abuser wasn't mad enough to section, and wasn't directly attacking me.
I thought they could have pushed for a harassment charge, but it was messy and I think they just didn't really see how stressful it was for me, sympathetic as a couple of individual officers were. They see so much worse.
I don't think you leaving will make you happier to be honest, so I don't think it's the best thing to do.
I think your stepchild and partner need to stop telling you what she's up to, you need to get a break from her, your stepchild can still get support from his Dad, but they need to leave you out of itball.
You need to tell any family members not to pass on any information about her to you. Once your stepchild is 18, your partner needs to do the same.
I suspect your stepchild will break with his mum when he's an adult, so she's not going to be able to get to you in a few years.
If you live anywhere near her, I think you should move.
The non-molestation order is a good idea, but don't give up hope if it doesn't work.
Keep a record, tell the people who need to know what's going on - in my case it was GP, school and my work, as my family member had written to all of them with false allegations that I was abusing my kids.
You have all my sympathy, you'll get through this.