Op I remember your previous threads. I'm sorry things have got worse.
I suspect that "John" is a survivor of sexual abuse by his own parents and therefore sees abuse in every family, even when denied by the person he's actually counselling (as in your case when he accused your parents of neglect.)
The problem is, people in abusive relationships DO frequently deny that it's abusive, because that thought is too scary. So it can be tempting, if you like, for therapists to blame every negative emotion and difficulty on an abusive past. It can also be tempting for the person in therapy to believe this - because if I believe that all my problems are a result of my dad sexually assaulting me, I can just cut him out, deal with the abuse, and everything else will be okay.
My dad sexually abused me for years. I struggled to repress the memories. There were some things I did successfully forget until they popped up in counselling. I have no problem with that concept. There are doubtless hundreds of photos of me as a child looking happy. Abuse victims do not spend all day every day cutting ourselves and crying. We are allowed moments of happiness, and family ocassions such as parties can actually be a respite because you know your abuser won't be able to assault you for at least the next few hours. So I wouldn't put stock in that one way or the other. (Unless you mean she is claiming that there would have been obvious visible injuries to areas not covered by clothing? Sorry its not clear from your posts.)
But as you've said, the problem is with things that demonstrably did not happen. Then it becomes a belief in some huge conspiracy. Certainly there have been cover-up operations, strings pulled, witnesses discredited, in many cases. Just look at the Catholic Church. Or Jeffery Epstein and his very convenient suicide. So it's easy to understand how your wife could fall into this trap. And then if you speak out and say "this is bollocks" it's very easy for "John" to tell her that you are part of the cover up. That "they" have got to you.
And from there it's a short step to her accusing you of abuse of her and/or your son.
I'm wondering if there are organisations for families of people who've become believers in things like Lizard People, 5g causes Covid, Covid doesn't exist, etc? If they could offer advice on leaving safely and protecting yourself and your son. STAY AWAY from anything regarding false memory syndrome (FMS) because IME these groups are often organised by actual abusers, with obvious motivations.
The situation cannot continue and I'm glad you intend to get legal advice.