This is more in response to your original post than the IC branching, but I've been mulling it over for a while.
I think I see some of my own story in yours, and since your husband seems like he wants to reconcile, I’ll share what has worked for us in hope there's something in there that helps. (I’ll note that our story is far from complete).
Here’s what I see: You engaged in relationships and sex that was secret, illicit, rule-breaking, and totally against what is acceptable in your community. During that time, you met your husband and had an immediate attraction to him. But he fit the norms of your community. You wanted to be a person who could be with him and you wanted to leave behind the other person, so you hid what you had done and what you had felt, super-compartmentalized, actually forgot, mentally moved on. It was hard to do that on a dime - you had a links to the past (the EA that you brought in to the marriage) but that stopped after a while. You thought you had successfully left behind that part of your life and moved on. And you were so used to hiding that part of yourself from your husband (and from yourself) that it didn’t even feel like the “you” who married your husband doing that.
That’s parallel to my story in some ways. The sexual life I had before I dated my husband was something I wanted to hide and forget. I didn’t think it had any place with my husband, who seemed to follow all of our community’s rules so successfully with nothing of his own to hide. I loved my boyfriend/husband and wanted so much to be with him, but I thought that the part of me that engaged in those impersonal, emotionally violent sexual interactions had no place with him, so I put it away. I gave him a vague cover story that had some poetic truth to explain my eyebrow raising reputation, and he tacitly agreed to not ask anything else because I signaled it was off limits. I thought that part of me would shrivel up and fade, and I only showed him the part of me that I thought conformed to what he wanted to see.
So you and I both had sexual lives that were not acceptable to our communities, met a “good” man that we loved, were worried and ashamed that we had feelings and impulses that we thought were not able to come out around our husbands, and stuffed the feelings away to we could get on with our good lives. Our husbands worry that they are inadequate/Plan B because they think our “real” feelings are the stuffed away ones; we think they are off-base because those feelings are bad and we are trying to get rid of them and leave them behind.
I’ve finally allowed him to see and know everything, and I’ve been shocked – and relieved and excited – to find that my husband actually likes all the crazy parts of me. He loves them, in fact. Not the affair. But the intense fighting way I was trying to deal with my life, all on my own. The violent sexual power plays – he LOVES that. He's not such a rule-following guy himself, it turns out. We find ways to elicit it, within our marriage. He feels so much more like a man, Plan A, all I want and need. And he is sweet and gentle and loving in talking about the hurt that caused all of that craziness, to be part of the healing that will allow me to not be driven by those impulses but to understand them and manage them.
I’m sure there are many reasons your husband is spending frenetic energy checking into your past but at the root I think he wants to know who is this woman I married? Does she really want me or is she pretending? And as he discovers more of who you “really” are I think he’s despondent, in part, because you are not allowing him a place all parts of you. But I think he’s also fascinated with who you are. He probably sensed a rule-breaking side of you when he was first attracted to you and loved it. He may want MORE of that for himself. I think he wants to know that side of you, and he wants to share that with you. I wouldn’t be at all surprised if he is thrilled by breaking rules with you in reality and in fantasy. So part of creating that timeline for you is giving him a gift of the full you, not warts and all, but fascinating nooks and crannies. Don't be ashamed of it - own it! You were not some rule following patsy*, you wanted to live! And then figure out how to give him ALL of you in the marriage. Tell him just how “bad” you are, what your secret passions are, and you may find that, directed at him, he doesn’t think it’s bad at all. This is what will help you be honest and not hide. It will be reassuring to him if he is able to know and meet all of your needs, and it will feel healing and make you more in love with him than ever if you feel like the whole Change4theBetter is known, accepted, and loved.
*note - one problem with too many rules is that the dumb ones that you were probably rebelling against (good girls should not be sexual) get tossed out with the really really important ones (stay away from married men). Your husband can you figure this out.