Gywn, first of all, I want to say how proud I am of you. You've come such a long way since you first posted here, and have made great strides working on your anger and inner strength. Bravo!
H doesn't want to bring up the A likely because it causes him to face his guilt and pain and he knows it will cause you pain as well, and doesn't see the need for it to be discussed. But you and your C do see the need, so let the C be the ringleader to open up those discussions. Perhaps the C can explain why it would be beneficial to your healing, and why it would also help in understanding what went wrong so as to help prevent such a thing from occurring again and make better the things that went wrong.
The only part I'm skeptical about is that H comes up with the reasons... I mean, obviously, he has to be the one to talk about it, so it's really going to depend on your and your C's ability to delve into the psyche here to explore and find the real reasons. For example, H may say that one of the reasons he got involved was because there was a damsel in distress, and as a compassionate human being, he stepped in to help. Well, that may appear as a reason, but it's superficial. It certainly illustrates the social environment in which he may get involved in an affair, but the real reason under his co-creating that social environment may be: he feels a need to fix other people's problems, or people having problems annoy him when they can't solve them, or, he feels no one can do anything right without him, or who knows what... his "rescuer" tendency would certainly be a problem to address.
Of course, just in helping other people, for whatever the motivation, one doesn't end up in an affair. So even a "reason" such as the above given needs to be discerned as to whether it's a key to "but why the affair?", not just the social environment that was conducive to the affair.
