I agree with Pen. What would you be trying to elicit from him? Asking why is a real cheeseless tunnel. The important thing is to move forward together and work on your NEW marriage. NG is making clear efforts to prove he is committed to you and your new life.
I think that the big difficulty is HOW do you build a strong marriage? I am hopeful that you will figure this out and have it all tied up in a bow ready to deliver to me by the time my H returns home.
I really can understand the lack of trust you feel. Will knowing WHY this happened return that trust? Does NG have a clue why he acted out? And despite the fact that you may not trust him, do you trust yourself a bit more? Do you have confidence that you will take the steps you need to take should anything arise? I believe you will. Trusting yourself to stay or go, trusting yourself to know your own heart, is a very big plus.
Your situation seems so unusual to me because he seemed to have never wavered in his belief that he wanted to remain with you. When you say you have your own understanding of why this happened, does it include the possibility that very little had to do with you?
I run the risk of feeling or saying that you are lucky in your situation...yet I know that the distance and the anger and the loneliness you have felt have been excruciating for you. And I also suspect that, should my H ever decide he wanted to return, I would be saddled with the same set of questions and the same doubts and, I bet, the same anger that all of it had happened in the first place.
The only thing that I can relate that may be of some use, is that Michele does say in the DR that they rarely apologize or are as regretful as we would have them be when they come back to us. That tells me that, for whatever reason, our spouses had to do what they had to do.... and it was our decision to hang in and make it come out differently.
I lift my glass to you, Slowly. It may not feel like it yet but you ARE a db success...and they tell me, it DOES get better. Sounds like your situation is already turning in that direction.