1. Ok so you don't believe her, and trust either of them. When will you? The guy is a dirt bag. There is no doubt of that. Your W has not done much to make you feel better. Yet this is your chance to set a benchmark. To get some closure. Go ahead and share the letter with your W. Make sure she understands how you feel. Tell her you want to believe this is it, but that she needs to win back your trust. Always emphasize that you want to trust her. When you do tell her that the hard work is on her court make sure, you don't say it in anger, or even passive aggressively. Make sure you explain to her that this hurt you. Don't make it an attack on her integrity or she will just resent you.
2. As for busting this guy, it will do nothing for your marriage but bring you trouble. It feels like justice but it's really revenge. Remember you are the good guy, he is the dirt bag don't lower yourself to his level. That being said, if you do your wife will most likely side with him. Remember he's the poor twisted soul, who needs your W support. You are Mr Low. on top of that you destroyed his life by outing him! How dare you! I'm sure it must be infuriating to read this, but I think we both know how twisted WAW thinking can be.
3. Finally I can tell by your post that your W is starving for approval, and attention. This low life gave it to her. Rest assured other low lives will too. I'm not saying this to get you paranoid, but to make you realize that you need to find out what she is getting from these OM that she is not from you. Where you can provide that for her do, where you can't make her understand this is who you are.
4. I understand she may not be full blown bi polar. I don't think mine is either. Yet as we both said they have some serious tendencies. I know you must be really hurting, but remember that smile. You have to be Mr realistic high.