JB42, your sitch is very similar to my sitch. (I read your sitch on Friday but I haven't been able to respond until now.)
Yes, the EA doesn't surprise me at all. 99.999999% of all of the sitches here involve another person in one form another. Even if it is just an fantasy of an OP. Many of us have written extensively about vulnerable to the charms of an OM wives in our situations are (OVR was alluding to this when he mentioned I'd be along to confirm this).
But here is some good news: It changes NOTHING you should be doing. GAL, 180s, detaching. Being the best JB42 you can be. Be the man only a fool would leave!
Right now the best piece of advice I can give you is to not lift a finger, in anyway, to help her fast track this divorce. You don't need to actively do anything to stop it, but definitely make her do the heavy lifting. She will have a fantasy fog of a perfect, stress free divorce. Do not give it to her. Make her file. Make her do all of the legwork and document gathering. Make her do all of the work. Likely she will never even file (even if she moves her EA forward into a full blown PA). If she doesn file, hire a lawyer. WAWs HATE getting lawyers involved. It interferes with their fantasy of a perfect, roadblock free D.
My W even wanted to give me full custody, and not try for alimony or take half of the marital assets. Why? Because in her mind this made it so there was nothing to fight over. When I explained that we would be splitting everything 50/50, all assets (my retirement accounts), current account balances, selling the house and splitting the equity, sharing custody, and that I would also be paying child support (and potentially alimony depending on how quickly she found a job), I could see her fantasy bubble burst a bit. Then when I told her I spoke to an attorney it burst further. You guys have kids, so there is not such thing as a stress free D.
So make her do all of the work. But protect yourself. Please do not come back and say "but I can't afford an attorney". That is never the problem...the problem is ALWAYS that you can't afford to NOT have an attorney.
Read my sitch. Read other's sitches. All sitches have their differences, but many are very similar. I think if you double-down on the DBing, especially detaching (let her go to get her back!), you have a good chance of turning this thing around.
P.S. Forget the OM. Your problem is not with him. Trust me on this, if it wasn't him it would have been someone else! In fact, even if it were to end with him today, she'd be on the look out for the next OM immediately.
M(53), W(54),D(19) M-23, T-25 Bomb Drop - Dec.23, 2017 Ring and Piecing since March 2018