[quote VenusTiger]@Pottedpalm it's not "utterly ridiculous" watching your CHILD being bullied and abused right in front of your eyes.
I've said I would do this - you're damn right I bloody would, the GF needs exposing big time and I would do it in a way that only proves who she really is as much as possible without any violence of any sort.
She needs to keep tripping over herself.
That is not ridiculous.[/quote]
Sorry but it is a ridiculous idea.
There is no excuse for using tactics worthy of the GF and there's a high chance it could backfire massively - such as the OP being accused of harassment.
Equally if she behaved in ways similar to the GF you both normalise and legitimatise it to the son.
It's a hugely frightening, frustrating and emotional situation, which is exactly why a "cool head" has to prevail.
At 16 he can move out and from the sounds of it his GF's parents would offer him their home - at which point the OP loses all possible ability to keep tabs in his well being and offer an alternative narrative.
OP, given your updates I think you're right to back off. Upping the ante at this point feels counterproductive if the Police can't be of much assistance.
Let him see her but back off from the drama. You're done. You don't want to talk about it. He's chosen this and you're finished with picking up the pieces and consoling him.
He doesn't want to do his sport/see friends/go out with family - fine. Focus on your other children and crack on having a good time without him (hard I know but don't let it show). She's still not welcome in the house though.
I get the feeling the GF enjoys butting heads with you as much as controlling your son. You can at least deny her one aspect of pleasure in this.