I am dealing with a nutty girlfriend too. Similar to your situation but not as extreme as yours.
I think you are on to something when you said they want to get a reaction. If they did get a "bad" reaction" she would use it to destroy you. Therefore be that actress and act totally none flustered by any of her antics. She may ramp it up for awhile though.
Also, have you thought about keeping a list of the social service visits? Like a speadsheet? That way, the next time they show up, show the social worker the spreadsheet and have a space for them to sign it. Tell them you are keeping track of all the false accusations. That might come in handy one day in court.
The nutty GF in my case, is alienating/brainwashing my kids. I have noticed as the kids age they become more aware of it. I try to model good parenting all the time. If they come out with a lie from GF, I show them the proof that it is a lie. My kids are 17 and 18 and I have shown them the separation agreement so they can see for themselves what the truth is. I also promise them I will be truthful and transparent with them.
Finally, I realize i am being persecuted by GF. There is nothing I can do to stop it so I work on detachment a lot. I pray, a lot. In just a little while longer the kids will be emancipated (age 19) and the CS issue will cease. And I try to have FUN!!!
And, thank God I am not her.