First I want to tell you that you handled the confrontation good under the circumstances, and that of course it's unrealistic to think you would get most of the truth in one go after yearss of betrayal.
As others have pointed out, the fact that "he suspects the kids are his" statement implies that OM more likely than not didn't know of/or agreed to any form of "sperm donor"scenario, plues she did it TWICE with him years apart fromt the first time (confirms LTA), the more likely scenario is they had an A, OBS found out and he dumped your WW like a hot rock in order to save his own M, she then hated him/them for the outcome. Also "stalking OM/OBS" on social media is typically a sign that she possibly still has lingering feelings for OM, instead of being afraid of those "dangerous and crazy people", what else could they do to "destroy your family"? tell you the truth? the same truth she's been hiding form you ? but you know this, your WW destroyed the family not them, they're not married to you she is, was blackmail involved ? maybe but that was after the fact.
I don't know how this M is salvageable, I always recommend D when it comes to LTA and serial cheaters, btw your WW admitted to trying to start an A with another OM so I would press her on this, there may be others, after all you mentioned that many other people got fired at the same time back then, however if you insist on giving R a chance here's some of the basics to improve your chances of a successful R that have stood the test of time:
1) Timeline: A complete written timeline of the A(s) is in order followed by a polygraph. If you are to forgive you at least should know what you're forgiving, this is a life altering event and the more info you have the better.
2) Full Exposure: At this point you don't even know when this LTA ended or if it ended at all, she's been stalking OM/OBS, a typical sign of lingering feelings, full exposure with family and close friends typically kills the "beautiful, romantic and exciting" aspects of an A and replaces them with pure shame and embarrassment, the more she hates her huge betrayal the more she will hate what she did instead of justifying it with the "I did what I had to do for the family". I also want to emphasize the fact that children have the right to know, for medical, honesty and moral reasons, they don't need all the gory details, just the facts, make sure you have a good therapist on stand by just in case, also you certainly could use IRL support from loved ones in your corner.
3) NC Forever: She needs to send an NC forever text to both OM, one that you approve and watch her hit "send", short and to the point (no sweet goodbyes). If child custody or chid support issues arise then those should be handled by your attorneys.
4) STD Test: Demand she gets tested for STDs/STIs (you should too), some STDs/STIs remain dormant for years, early detection is usually vital for effective treatment, plus the infamous "walk a shame" to the doctors office could help with remorse.
5) Full on demand access to her cell phone and all electronic devices and passwords FOREVER, no questions asked, don't accept the typically "it's my privacy" excuse/defense, the "my privacy should become "our privacy" when you tie the knot.
6)VAR: Voice Activated Recorder, you can easily order one oneline, about 50-75 US dollars, place one in her car under her seat, right now you should be deploying all available resources to try to get as much unfiltered truths and facts as possible, cheaters typically talk in their cars to APs and/or friends/confidants about their situation.
7)Therapy: She needs intense IC to find out her "whys" with someone who specializes in infidelity (very important), forget MC for now, at this point MC is usually a waste of time and money.
8) Consult a D attorney to know your legal options, while you're at it, ask about an enforceable post nup in case you later decide this was/is a deal breaker for you or she cheats again.
Again I still suggest D in your situation after such as daunting betrayal, your M has been a farce and your WW a fraudster, but I want to also emphasize the FACT that your children are not biologically yours, but you ARE their dad and they ARE your children.
Keep posting frequently, the collective wisdom of SI could help you go through this difficult situation.
[This message edited by Buster123 at 8:52 PM, Saturday, February 5th]