asteria I've called the NSPCC and SocServ myself about specific concerns I've had regarding my DSC in the past. Don't phone them with a list, but the next time something happens that you are really worried about (you know they've been left unsupervised, they have unexplained injuries, or are just displaying emotional distress because of the situation) call up Soc Serv and ask for their advice - you're not sure if you are right to call, but you are worried about your stepDCs welfare Or report to the NSPCC online as those reports are forwarded on to the relevant SocServ dept.
It will be a slow drip, drip drip into the system. There's no point in telling them about the catalogue of things that have happened so far; that will be seen as bitter ex-spouse territory; but if something bothers you, ask an expert whether THEY think its of concern.
My DH ex is similar as I've explained - her reaction to DH interfering with his DS schooling choice was to apply to court to have the current court order changed to prevent all contact between DH and his DS. She never expected there to actually be a hearing, and when the court date got sent to them she phoned DH up in a panic, saying that it had all gone too far and it was all his fault because he didn't do what she said.
She initially presented well in court, and the CAFCASS Officer was quite hostile towards DH, but then when DPs ex was Faced with the Schedule 2 letter (given to her in court) she lost her temper with the CAFCASS officer because all the calls to SocServ that DH has made over the years are recorded in it. Her reaction to that was the first indication to the professionals that something wasn't quite adding up.
She is self-rep'ing, and when asked to present her case at the first hearing, said that she hadn't expected to have to speak - and then spent 10 minutes ranting, accusing DH of abuse and harassment. At the next hearing, not only were these accusations dismissed by CAFCASS and SocServ, the court ordered them both to attend the Separated Parents Information Programme. She responded to the magistrates that she didn't see the need to. Needless to say, that did not go down well. We find out in a few weeks if she's actually done it or not!
What I'm saying is that not only will your DPs ex eventually hang herself through her own actions, but her behaviour when things don't go her way will begin to reinforce what your DP will have been saying for years.
DHs ex has previously talked her way out of further SocServ intervention after she admitted assaulting her DD. i know whst you mean when you say that your DPs ex is incredibly convincing and credible. But, when under pressure because things aren't going her way, that's when she's likely to lose control in front of others - particularly if her behaviour is driven by an undiagnosed personality disorder.
Give it time, remain patient, support your DP and be gentle with each other! Accepting that the DCs will be damaged, no matter what we do was probably the hardest period of our relationship. But we survived.
I'll PM you some book recommendations that have been incredible useful for us.