I like how you are handling this and I think you're being very wise. By not badmouthing your wife it helps allow you to keep the "door open" to her for reconciliation. But I also agree that hiding the truth is not good. I personally thought it was important to tell people close to us, family members, my support system, and those I trusted about the situation. Also, once the divorce wheels are set in motion you can't keep pretending. The situation needs to become real. I think hiding things is wrong and affairs and poor behavior only flourish in secrecy. But to those I have told about the situation, I never described my husband negatively. I just explained it's a midlife crisis, oftentimes in a long term marriage people take each other for granted, sometimes people have to lose something dear in order to figure out it's value, and I'm keeping the door open to my marriage up until the very end. It's not hurting me to hang out and it's giving me the time to heal so if I go into a future relationship I won't have the extra baggage.
Sometimes people would question me on this. Many of my friends dated during their divorces, but I would say... I'm still married until those last divorce papers are signed, I'm in GALing, healing, 180s and "IT'S ALL ABOUT ME" mode (A healthy ME mode! I've always been the caretaker, giver, fixer and had lost myself in the mommy role so I needed to focus on me a little more, my dreams, things I want to do with my free time when the kids were with him, time thinking, reading, taking myself out, shopping, etc...) and I definitely didn't want to get into any rebound relationships or add more drama to my life by getting involved with anyone. There's plenty of time, and even when I did meet a few attractive neat guys who were interested in dating, I figured they'd only think more highly of me if I waited until the divorce was final to get involved. I think it shows good character. If I'm treating my spouse well during divorce and showing respect to the relationship then I'm going to treat them well and show respect for them.
Anyhow, enough rambling!!! All long term marriages have "checkout times." Between kids, careers, everyday life we all tend to neglect our marriages at times. I think that's extremely common and it's easy to point that out and say it's the reason for an affair or the deterioration of a marriage. The results of this is just a wakeup call to change things and make the marriage a priority...but you know that....
People seeking divorce come up with a lot of excuses for things (even in relatively good marriages.. maybe yours was worse than some but I'm sure it was better than others... ). Your wife won't realize it's an excuse until the dust settles and she has had time to think.
If the MC is ethical (and most do try to be) she won't take sides or gang up on you with your wife. If she's a decent MC she'll just ask questions and let your wife or you talk. She may point out things, but typically when a spouse is invited to a MC session the MC is pretty neutral. If not, just sit and listen... no defensiveness... don't take anything personal.
With your marriage you do have at least one strength... your wife is religious. There may be others... look for those and see how you might be able to untilize them to hold your marriage together in a VERY SUBTLE way!!!! (note the very subtle!!! That's important or you push the spouse further away) In my marriage the strengths were that my husband values family, I was a great listener and friend to him, he's very attracted to me. These were advantages I used in a subtle manner to slowly try and pull my my marriage and family back together.
There is no arriving, ever. It is all a continual becoming.