I don't think you are ever going to know which one is telling the truth. Ok, so he may be scared to admit he lied to the nurse, in case it drives you away. On the other hand, she isn't going to sound too good if she says 'oh, yes, I was deliberately trying to break up his relationship with you' - sounds much better if she plays the innocent victim who thought your relationship was over.
Either way, he is the one who has a commitment to you & he is the one who has been unfaithful to you. No one can force another person to be unfaithful. I'm not sure that investigating further will help. The question you have to answer is whether or not you can rebuild your relationship in time and move on. As CLKD says, it might help to think how you would feel if the option to rebuild were taken away from you.
Hope you are able to work things out and that the new year is a good one for you.