Crushed, let me just say a thousand kudos to you for how you've handled this so far. Although everyone and their mother says they wouldn't take shit from the WS if they found out that their spouse was cheating, the vast majority of us don't.
You, on the other hand, haven't accepted his lies, made him leave, consulted with a lawyer to get a clear picture of what D would look like for you, and are putting the onus on him to prove himself to you and win you back. Most importantly of all, you haven't gone straight to marriage counseling; you're starting with IC first. That makes it clear to your WH that this is entirely a "him cheating" issue and not a "something's wrong with the marriage issue." Further, during these early stages, you need someone who is going to give you advice based on what's in your best interests, not in the interest of staying together.
Will all of this guarantee that reconciliation will be successful and that your husband's cheating days are over? No. But you have a better chance than you would have if you took him back unconditionally, pick-me danced, allowed him to weigh his options, and/or allowed yourself to be petrified by fear of taking action.
I think all of the things your husband is doing and, more importantly, the fact that he's doing them proactively, is a great start. The question will be whether he can sustain this level of intense effort in the long-term. So here's my practical advice:
(1) Take him up on his offer of a post-nuptial agreement ASAP. Strike while the iron is hot and he's desperate to win you back. Don't give him time to second-guess himself or run it by his friends.
Also, keep in mind that in order for a prenup to be enforceable, it must be fair and it must be reviewed and signed-off on by a separate attorney hired by him. So don't go into it demanding EVERYTHING. Prioritize the things that are most important to you, such as protecting your pension, retaining sole ownership of a specific asset (for example, a piece of property that's been in your family for generations), or not paying alimony.
(2) If he's in AA and NA, then obviously addiction is something he struggled with before the affair; this woman was likely a stand-in for booze or drugs. Before agreeing to attempt reconciliation, I really do think you need to seriously consider what his addictions have already cost you emotionally, mentally, financially, and (possibly) physically.
(3) Get tested for STDs. Both your husband and OW were engaged in high-risk behavior, even before the affair. The most dangerous lie you could possibly believe is that they were using protection.
[This message edited by BluerThanBlue at 6:51 PM, Tuesday, September 17th]