You finding out right after it happened would have hurt, finding out about it decades later hurts worse because it feels like the relationship has been built on a lie. How did he end up in a different cart on the carnival ride? If you went to the fair together that doesn't make sense, unless you were with the kids? Did he know who she was before he sat down with her on the ride? If so, what the hell was he thinking?
So his story is she gave him her number, he called hoping to speak with her dad about the odds of your husband and his ex-girlfriend meeting at the fair? If I am understanding that correctly it is laughably BS. My theory is he wanted to talk to her and his cover story is he wanted to talk to her dad
What is his job that keeps him out overnight somewhere yet he can sleep? And she didn't just show up on accident, he told her where he would be. Now I am making an assumption but how else could she have randomly showed up and he just happened to be there on an overnight shift? My BS bell is ringing LOUDLY.
I have a suggestion. Tell him to write out a complete timeline as best he can remember, leaving out no detail no matter how small. When he is finished ask him if this is everything and if he says yes then ask him if he is willing to take a polygraph test. If his immediate response is yes then there is a chance he was honest but if he says no or he stalls or starts asking why, then you can probably assume he wasn't honest and only wrote enough in hopes it would pacify you.
The general consensus is that it takes two to three years for a person to process through learning about infidelity and years beyond that slowly trying to rebuild trust. But when the BS has lingering thoughts as to whether or not the WS was completely honest, R is VERY difficult.
I echo the suggestions that you find and IC that specializes in betrayal trauma. He also needs to find an IC to help him fix himself. Please do not try to repair the relationship on your own, he broke it, and now it's up to him to do the heavy lifting.
A truly remorseful spouse will drag their naked butt through miles of broken glass if it would help repair the damage the/she has caused.
This forum is an amazing place for advice and support. I wish I had found it sooner.