I understand how you feel. You want to know everything that happened because it feels like a betrayal to you if you don't know. You feel that as long as you don't know, it's their secret.
As long as it is truly over now, though, that's the most important thing. One thing that I feel compelled to say to you is that it seems to me that she wants to keep it going in your mind. Don't allow her to do that; don't allow her to steal your good times with your H. Why would she say all of this to you, unless she wants to cause trouble? Maybe the reason she wants trouble between the 2 of you is that she wants him back? I would certainly question him about it, as you have, I would watch for signs of the 2 of them communicating, but other than that, I would let it go and show her that it didn't work. More than that, I probably would tell her it didn't work, and to get a life.
Sorry to hear of your dilemma. As one who's BTDT, I think the warning signs are fairly clear.
1. He lied about his engagement to OW
2. He lied about the car payments
3. He lied when he said, "that's all there was."
(Then you got married a few months after the above)
4. He's lying about the PA and, deep down, you seem to realize it.
4. He's lying about their March rendezvous and, again, you seem to know it.
You "think he's lying" because he is. Always trust your gut feelings and instincts when it comes to DH and his words/behaviors.
His resolve to not discuss any part of this is what you should keep in the forefront. You might want to be more proactive on this issue, ie. insist on getting into joint counseling or whatever it takes. Waiting for him to enter therapy for PTSD and 'hoping' that he'll admit the past may not work out that way. Take control; don't assume that he's going to have some epiphany and come clean.
He has lied to you about a lot of things. It is hard to start a marriage on a firm foundation when the foundation is cracked with lies. He lied about the engagement. He lied about the break up.