I've been reading through this thread, which is so long, unfortunately because that means a lot of issues - a lot of things have been covered up through many years and just wanted to post a couple of thoughts I had about cheating from long ago.
In general, I think when someone finds out their spouse is a serial cheater or has been in one long term affair, possibly for years, or has been cheating on and off since the wedding or before (surprisingly common) - I think the best solution is almost always divorce. Even with kids. If you stay together....it's gonna come up again, whether because of further instances in the future, or because as people get older, as I am now, you have the time and inclination to assess your life and perhaps the biggest area is your marriage and other partnerships. If you knew about shit being done years ago....it WILL come up again in your thoughts as you evaluate your life. Not just about cheating but about all kinds of things. We can't change the past now, but reviewing and analyzing it is something many if not most people do when we get old. I'm doing it now about various things. I'm kicking myself now over a lot of choices. My main advice about serial cheating (because a short term one off or ONS might be recoverable, R might be possible) is to get a divorce at the time, as painful as it is. Even with kids. It's better to take the pain at that point when you're younger and have time to repair your life, make other future choices and maybe a better 2nd marriage than to try to rehab a marriage that later on, reviewing it is going to give you a lot of grief. Divorce usually IS the best option. I know that doesn't help people who didn't, but for young and middle aged folks going through it now, you really should just get a divorce. The pain of regret in your old years is one of the worst things in the world, IMO, and there's nothing you can do about it, you can't change that past, you have little future now, you're running out of runway, and often you're stuck with a mate with health issues, etc, that you probably now think you should have divorced. Don't get trapped. Make the most of your youth and health AND DON'T SETTLE if you can avoid it.
SHOULD A WS TELL YEARS AFTER THE FACT?
For me, I guess it would depend on how the marriage was going at present time. If the marriage is going well and has been for a long time and the BS doesn't seem to have any issues or questions, I would take it to the grave. I agree with FormerPeoplePerson. If things are going well, the BS doesn't seem to know or suspect or it isn't causing issues, why wreck both of your old ages with this. Yes, it's lying but...what good will telling the truth do? It's only going to wreck what's left of both your lives. I don't think anything positive comes from that. If however, a BS knows or suspects or it has been opened up before and WS knows this is still an underlying issue - yes, you might as well go back and clean it up if you can. Tell them what they want to know. Don't have any expectations that anything positive will come from this though - revelation doesn't equal happy endings. It just gives the BS confirmation and validation of things they thought or suspected or knew anyway, which might be extremely important to them. But if they don't know anything and all is quiet and going well - I'd take it to the grave. Which leads me to my next thought:
WHAT THE WS IS LEFT WITH AFTER REVELATIONS
I think Cheaters probably reveal things after a long time to unburden their conscience, especially as they get older, you're alone together, kids are gone....and old age, bad health, death and judgement become impending. So people unburden themselves. If the BS already knows or suspects, it might help, but I'm reminded of what the Doctor tells Scarlet O'Hara when Melanie is dying "now don't go telling her anything that's not going to make any difference now - Miss Mellie's gonna die in peace." I understand and respect that, but I think people want to ease up their own conscience.
A lot of older BS will look back at their marriage and family life mournfully because they wonder if they ever had a real marriage, if they ever knew every thing, if their spouse ever loved them, they no longer love their spouse, etc, etc, but I think the chronic WS also faces a lot of issues in old age - if they have any sense of decency or self respect or love or affection for their spouse, I do think many experience regret if not remorse because they can't sugar coat what a shitty person they've been and how badly they've treated their spouse. They may not say this openly to the BS, but I think it's often there, not even that openly expressed but the hidden sense of.... I have been and am a shitty person. I've done bad things and hurt others, especially my spouse, repeatedly. I have cheated with different people, I broke my vows, I deeply hurt my spouse, my kids may not respect me. I don't respect myself because there's not much to respect. When you're lived your life in a shitty, deceitful, promiscuous kind of way, at the end of life, that's what you are and that's what you're left with. It's not a good feeling I'm sure. Any of us who's ever hurt anyone earlier in life and they died, whatever the hurt was, or knowing you don't have that much time left....that kind of regret is incredibly painful because you can't make amends. Even telling the truth does not make amends. You can try to redeem yourself through Jesus or some such but it's not going to change what you did to your spouse and the relationship you end up with. That's the legacy they leave to THEMSELVES. The WS is not necessarily getting away with ANYTHING - they might actually have a larger burden to bear of guilt and regret. They may think their spouse does not love them....and that's often true by that point. Think of Jacob Marley in the Scrooge story - he expresses something like this to Scrooge - it's his motivation to help Scrooge become a better person while there's still time.
So....deal with it permanently when you're younger, if you can. Don't live with the grief because it only gets worse when you're old. Live your life in the way you want to be remembered for, and look back on with pride.
Sorry for the length of my posts, I know I can be tedious.