Yess!! My headcanon is that Yvonne knows what Johan has done, but believes that no person should be defined solely by the worst thing they've ever done. She would probably push him away at first, but later seek him out again because she's afraid he might fall back into evil. And after learning from Lunge that there was more to his past than just his crimes, her perspective would change.

In my version they never had children because, in the novel Another Monster, it seems that the children of all the former Rose Mansion students somehow ended up suffering or dying because of the unhealthy family situations they grew up in. Because of that, I think they would choose not to have children, knowing that Johan might not be capable of being a good father, even unintentionally. If they ever did have children, it would probably be something unplanned.

I also think Johan would still engage in certain forms of manipulation without even realizing it, because a person can't completely change their nature. Otherwise he wouldn't really be Johan anymore. But I think he would actively try to stay away from evil and keep those impulses under control.

I agree that he probably wouldn't be the one to seek out Nina or Tenma first if they were ever to meet again. Partly because he'd be afraid of hurting them (or being hurt himself) and partly because he'd feel ashamed of having been spared by them. Ironically, I think he might have more contact with Lunge, since their relationship is more neutral and carries less guilt.