I understand what you're saying Stella, but I also understand that under pressure people don't make perfect decisions, they need help to plan the possible outcomes of scenarios based on their intimate knowledge of the person they are dealing with, by someone who knows how to deal with these situations.
Whilst I am absolutely sure you are right about how the legal system views abuse situations and what they require the target to do and how they require the target to behave in order to prove they are actually targets not participants. I would counter that the law (because of the burden of proof) has unrealistic expectations that someone being abused always behaves with the perfect rational detachment of a disinterested on-looker. In the situation WTBH describes, stating that she will phone the police if he persists in verbally abusing her is the sort of advice that is given on here all the time, and when the OP describes what happened, blaming her for his massive over-reaction and physical abuse in front of their children is entirely unhelpful. That stance doesn't help keep people safe, it actually helps perpetuate the situation. That may seem counterintuitive (much as dealing with an alcoholic can seem counter-intuitive) but that doesn't stop it being true. I would suggest we all re-read the "about abuse" sticky. There is a vast difference between blaming someone for something that has happened, and sitting with them, validating that abuse is NEVER the victim's fault, and suggesting a plan of action if that situation or something similar happens again.
Further, in my case a non-reaction doesn't de-escalate matters, I have to have an entirely different strategy. It may well be the case given OP's ex's confidence that nothing she could have done/not done would have made any difference to the outcome (he has any times physically abused her AND had her arrested for it, when witnesses are around, when she has done nothing to "provoke" him. I bet in this situation he would still have punched her, the neighbours would still have lied, and she'd have had no different outcome, whether she had responded or not. Therefore she needs help and support to keep her and the children safe first, in her particular situation, and then legal protection to cement that (because frankly, in many situations the law only actually kicks in effectively after the abused person is near-dead). A DV centre can help her sort out strategies for her situation, help her practice them and give her the confidence that they will work, not tip him over the edge.
that is my experience.