This is a very sad thread. Ironically I have worked broadly in the field of family therapy and spent years holding my own family together, so I find it tough to have to resort to NC with my PIL.
Rusty. 'It's so hard to stay NC as I keep wanting to try and fix everything' This is just how I feel! Actually, are you me? It breaks my heart that they are treating DH in such a terrible way. yes, yes. And why does he still keep in contact with them?
No, Imbroglio I do not have to see them, thank god. They finally went too far, even for my DH and I will never agree to see them again.
They have been instructed to not ring the house or visit and for the last 6 months have stuck to it.
And no there are no children, which is actually the root cause. Turns out I am a recurrent miscarrier and now have a severe chronic illness (all discovered after the wedding). Like OP, there have been a string of accusations from PIL.
wizzy their list is long, colourful and unbelievable, just like yours! The worst being 'willfully causing my DH's childlessness' (they have e-mailed 'evidence' of this) and......wait for it.... even my own miscarriages
There has been much wailing about DH not having his own family (They have other GC) and -underhand- attempts to get him to leave me.
But, as Imbroglio says I am not shackled anymore and I will remember that I will not be punished anymore for setting and keeping my own perfectly reasonable boundaries.
wizzy I was doing a bit better until I read this bit from you i think low contact means that that door is always held partially open for a full relationship to start again
I think that's the bit I struggle with. I guess it's because the chronic illness/RMC has triggered anxiety and the thought that the door is slightly ajar and they might prise their way back in again gives me the collywobbles 
This is very therapeutic!