Revenge is sweet.
For quite some time you have wanted the the stbx-wife to confess her affair to your 8, 10, and 11 year old girls... Stbx-wife is forever shamed in the eyes of her girls - and this would crush any mother - but a dxBPD mother will be broken by it. It is as damaging to her as the affair was to you - maybe more so. And she won't be able to escape the shame - this will resurface for years in disputes with the kids. Her boyfriend is also whacked - he will be a target of blame and this will play hard on their relationship and make it extremely difficult, if not impossible, for him to develop a relationship with the girls. They did it (affair) and these were always the possible consequences.
I allowed W to take the lead with regards to how that was managed and was content to leave myself neutral and allow her to take any fallout from the disconnect between what kids knew and were aware of and the ‘story’ W fed them. Clearly this didn’t happen and the kids sought my version of the truth because they trusted my integrity. They could not trust their mother because they knew for a fact after discovering their own evidence that she had been deceitful. Was the option to remain neutral still there? I don’t think it was since staying neutral and providing no information, or deferring to W to provide information is a story neither were willing to listen to anymore.
Respectfully, I can't agree with your rhetorical question. You had choice. You had choice to mitigate or intensify -
protect the children or revenge the wife's affair. In fairness, you got plenty of advice on how to mitigate without lying and you got it early enough to be able to do so. You also got advice on how to reverse course once the train was headed down the track. You
chose to intensify. You have to own that.
Your choice to intensify, no matter how justified (and it was justified), will have ramifications for years - now that the kids are fully immersed and participating in the adult fight and there is an adult competition for their approval (and disapproval of the other parent).
Research has shown that kids recover from a reasonably amicable divorce in 12-24 months and live life (academic achievement, emotional and behavior problems, delinquency, self-concept and social relationships) similar to kids from intact households. Researchers have also consistently found that high levels of parental conflict during and after a divorce are associated with poorer adjustment in children.
Technically its not too late from the children's perspective to resolve this, but now that the adult fight is fully amped up, and the prior attempts by the boyfriend to work amicably nailed shut, the battle lines are drawn.
A person with BPD feels wounds much deeper than a person without and they tend to have either a disproportionate and exaggerated response or they walk away. A person with NPD tendencies often reacts intensively to an ego (narcissistic) wound. They can plot and retaliate for a lifetime.