I'll let you get your bearings, but will keep checking back on your thread to answer any questions, Dagny.
But know this: "busting an affair", or "laying out (and enforcing) strong boundaries", and "save the M mode" . . . are NOT mutually exclusive!! In fact, every piece of "take a strong stance" advice I've ever given on these boards, has been with saving the marriage in mind. In over six years and nearly 20,000 posts, and studying literally THOUSANDS of affairs, I have personally never seen the "be their friend" or the "Little Bo-Peep" thing work.
Not once.
"Puppy's Short Version, All-Inclusive Advice" for Waylayed Spouses Who Just Got Bombed"
1. Get proof (of whether or not there's OM/OW). Hint: There almost always is.
2. Do either:
2a. Aggressive affair-busting (see Allen A's posts over in Infidelity)
Those are the only two things I've seen work. If #1 turns up nothing, then proceed to #2b. And in the meantime, GAL your ass off, and become the better option. Best case, you'll attract them back, and at a minimum you'll become a better person that will be more attractive to SOMEBODY in your next relationship.