I feel awful for you, and I'd like to help. But first, a gentle caution. You may never find the truth, regardless of who you cross-examine. Your husband seems just as unwilling to shatter the status quo as you do. He will deny it. The man next door said "I think" 15 years ago, and this is suspicion, not proof. The woman next door is back inside, by some understanding that we know nothing about. I doubt that either of them would care to have you open old wounds, particularly if, after years of anguish, they've finally begun to heal.

For your sanity, let's find you a way forward. I've found two ways.

Your narrative strongly suggests that you believe he cheated. Suppose, for a moment, that he had confessed back then. He breaks off the affair, and pleads for forgiveness. The other woman, rejected, leaves town. What would you have done?

If you can say without hesitation that you'd have divorced him, never to consider reconciliation, then I think you need to begin planning for the inevitable. Your post reads like a breakup narrative. I get the impression that you're staying only because you have a house, a business, and 2 kids with him, and you are scared of tearing it all apart, and starting over at 40-something, a single mom with two edgy teenage girls. You might like to plan your break for the day when your 15yo leaves home, say, 3 years from now.

If you think you might have given him a second chance, I suggest that you take a hard look back at the 15 years that followed. During that time, would he have regained your trust? If your answer is No, then it's over, and you can start planning your break. If your answer is Yes, your challenge, if you're ready to accept it, is to see if you can act as though all is forgiven. I emphasize the word "act." Your feelings won't be restored overnight, but it's a start. Reconciliation may come in many forms, and sometimes "restorative justice" (a period of atonement, penance, and rebuilding) is part of the deal; but 15 years is excessive. Long before then, the partner who was cheated on must be able to come to terms with it, and act accordingly. Since you have a 15yo, you might like to take the next 3 years or so to work on this act. That's more than enough time to find out whether you can pull this off, and whether it's having the desired healing effect on you, and on your marriage.

A final caveat: I'm no therapist, just someone who understands betrayal and reconciliation from personal experience. Hope this helps, and best wishes, whatever you decide.