You have gotten some great advice. I will try to answer your specific questions:
1) Does anyone have any experiences (positive or negative) of keeping the affair secret from everyone else, including family and friends?
Yes, I told basically 2 people when d-day 1 happened - two very old friends of mine. I am not very close with my family in that I don't talk to them about my personal life as far as my relationships go - never have - to the extent I can avoid it. In this case I met my now WH via my sister (he was friends with my sister's husband) so admittedly I purposefully avoided telling her as that would have blown things up with my family and with his friend-circle via her husband and I didn't want to deal with all of that.
My WH had a workplace A with a married co-worker. His AP's husband also worked with WH and AP and WH knew him very well - WH was in AP-OBS wedding several years earlier. AP also had a 6mo child when the A started. (So disgusting but I digress...). Even though I knew the OBS (he was a good friend of my WH - so I knew him but not really well) I did not tell him until I discovered the following year that the A had continued. At that time, roughly a year after I found out about the A the first time, I told the OBS. He was thankful I told him and admitted he wished he had reached out to me earlier as he had discovered something like 9 months before that the AP minimized (she "confessed" she had a little crush on my WH and that she had tried to take it further but my WH denied her efforts, which was true when it all first started but when OBS had caught her it was really a full blown EA/PA replete with "I love yous" etc). Basically had we shared info earlier we would have caught it sooner - a lot sooner.
So my advice is - as far as your personal circle goes - tell who you want and avoid telling who you don't. BUT - tell the OBS. They DESERVE to know. I did not listen to the advice on this forum for a year about doing that and I sincerely regret it now.
2) MM (and his wife) are both in our wider social circle as a friend of a friend. While I want to avoid bumping into him as much as we can, there undoubtedly will be times when this is not possible. Is there a conceivable scenario in which encounters with MM, such as in a bar, could proceed without undue tension? Again, does anyone have any experiences (positive or negative) of the person their partner had an affair with still being in their lives?
Yes - see above. In fact, although OBS and AP have now divorced, they all still work with my WH. I told my WH that I did NOT want to see the AP period, at all. If we were invited to some event and she was going be there that we were not going. If we were to run into her somewhere in town (which weirdly has not happened - this is not a huge place) we agreed we would leave, immediately. No discussion - leave. So my WH had to decline invitations to social events hosted by co-workers as she and OBS (prior to their divorce) would be there. Once everyone at work knew, WH stopped getting invited to anything so that pretty much resolved that issue on its own.
3) Related to question 2, WW has suggested whether I would want to talk to MM - I have declined, but I want to know whether this would be a good idea or not?
I "contacted" AP via email. I wrote a lengthy email and sent it to her, the OBS and my WH - where I laid it all out - the pain, the misery, what horrible people they were - the lot of it. But I did it at a point where I did not care about the outcome, did not expect a response, and did it solely for me to air my own personal grievances so to speak. It was fantastic. Her crying voicemail admittedly was the icing on the cake but I had no illusions she would ever contact me.
AP also contacted me several times via text after I had ousted the A to her OBS - blaming me for ruining her child's life by exposing the A and causing turmoil in her marriage. Needless to say, I did not respond to those (save one where I responded only with something like "I am not responsible for protecting your child from your decisions - you are. You failed. Try harder.") but enjoyed receiving them immensely as it just reiterated what an unhinged and delusional sad person she was.
4) Going forward, how can trust be rebuilt effectively? Despite assurances from my spouse, I find myself consumed by paranoia whenever she leaves the house. Striking a balance between allowing her freedom and managing my anxiety poses a significant challenge.
My WH has undertaken 4 years of individual therapy - not to discuss our relationship but to figure out what allowed him to so totally breach his own morals and beliefs - to figure out why he was so desperate for compliments and to be desired that he was willing to toss me and one of his best friends to the curb for a woman he constantly described as "phony," "not the brightest bulb," and "kind of lame" when she first started working there - years before the A. D-day 1 was 10/2017. D-day 2 was 10/2018. D-day 3 was 3 (or 4 - I can't remember now) of 2019. His IC started in earnest - about himself and for himself in the beginning of 2019. He still goes 1 time a month.
The change in him is CLEAR. He is much less selfish - or at least catches himself out when he gets defensive and irritated. He apologizes and means it - unprompted. He clearly wants to be a different person and now has zero difficulty talking about the A - anything about it (not that we do much anymore - it RARELY comes up ever). I do now trust him as much as I will be able to trust anyone I suspect. I have learned some lessons about undying and unquestioned trust and I realize I was very naive about it before the A. I had loads of anxiety (see my post in general today about letting go of all that) and it's all gone now and I think it really has been for the last 2 years. So it took me about 5 years to feel "back to myself" but this new version of myself - the old one is gone, and that's okay with me.
5) Finally, regarding therapy, would individual sessions or couples counselling be more beneficial? Our initial experience with a couples therapist was less than ideal due to the therapist not meeting both our expectations, prompting my decision to pursue individual therapy. My spouse awaits confirmation for individual talking therapy from her physician.
Start with IC - for you for SURE. And for her, but only if she wants to go and to figure out what is going on in her head that would allow her to do this. If she wants to go to bitch about your marriage and/or anything other than her own issues, it will help little (ask me how I know).
[This message edited by ThisIsSoLonely at 5:19 PM, Tuesday, May 14th]