I wouldn't be so sure. I for one do not want to hear about any of my partner's past lovers. That's a HUGE turnoff. Especially if time is precious. They would have spent time talking about anything BUT bM. Yes, he might be encouraging her to leave him if she wasn't happy but as far as her telling him in great detail all the things that made her miserable about bM, I don't see. And I say that with first hand knowledge of a couple of affairs I've had. You're more interested in your own courtship instead of exploring the negative aspects of past relationships. Nothing ruins the mood more than complaining about your soon to be Ex.
Yet, many people do the opposite, especially if they are
still in a marriage. Both affair partners in this case were still in their marriages. Neither had filed for divorce or managed a separation (although if they'd each gotten to the separate bedrooms phase, that counts, in terms of how I'd classify such an affair).
Sometimes the entire purpose of the affair is closer to "let's support each other as we divorce" and in that case, each party commiserates with the other for all the pain they're enduring in their marriage.
I still talk about my past relationship with my current partner, who completely understands that I have C-PTSD because of it, and go through phases where it all still comes back. While it's possible that your view is closer to the truth here, we just don't know. I find it hard to believe that Suzanne never spoke of Barry's meanness and if she didn't (ever) to JL, I don't think that looks good for the prosecution. The relationships that Suzanne and JL would be speaking of would not be past relationships.
And many adult bonds form without much attention to the "mood" that's needed for romantic-sexual attachment, many adult affairs are more focused on shared support and companionship. I have an aunt by marriage who began an emotional affair (that turned sexual) while my uncle was still alive (dying of cancer). I did not judge her. Ultimately, that affair turned into a 20 year long relationship (never married) and it was a complex relationship, it was forged as they both went through the deaths of a spouse due to cancer. She helped raise his son and he helped raise her daughter. They kept their finances separate (her idea) but she helped him out financially in many ways. It remained an "affair" from the point of view of most of my older relatives, up until the time this second partner (of more than 2 decades) died a couple of years ago. I don't think my aunt ever had a boyfriend/sexual relationship based purely on sexual attraction or romantic leanings, there was always something else going on.
At any rate, most of my friends who have successfully left abusive relationships did spend a lot of time talking about the abuse in the early days of all their next relationships - and many of those relationships transformed into very long term arrangements, indeed. It's okay to speak of the grief and the dilemmas of a disintegrating relationship and many people do. Suzanne may well not be one of them - in which case, your analysis is right on. Perhaps she and JL lived in a fantasy world where the consequences of what they were doing were kept strictly separate from talking about the people they were affecting (and possibly hurting) with their affair.
Perhaps both of them wanted to pretend that unhappy marriages had nothing to do with their longterm ("holding a torch") type of relationship. You could be right - but if not, then JL would have information about what Suzanne was thinking about Barry (she certainly seems to think that Barry was having affairs - but if she never mentioned any scary situations or DV to JL, that's a win for the defense).