I can hear your frustration with the advice you are being given, but keep in mind that we are all anonymous here and it is a venue where people can tell you what you need to hear but not might want to hear. My experience is that this is a rare gift in life, and in real life, most people will tell you only what you want to hear. You can choose to listen or not.
For what it's worth, I think you might be confusing what everyone is saying. My read on it is:
1. You should focus on being the man you weren't before and being the best father to your D and husband to your W that you can be. 2. Don't snoop and try to avoid "the jealousy blues" as it makes you into a negative person and it sounds like, pretty obsessive. Remember the woman who said the thing she loved most about her husband was how obsessive and controlling he was? Right, neither does anyone else. 3. That if things really are going to work out between you and W, she needs to end it with OM. That doesn't mean you should be heavy handed or approach it angrily. And, it doesn't mean you have to talk to her today. But, my sense is that probably in the next week or two, when the time is right and you can approach it calmly, it would make sense to say something along the lines of "whatever happened before happened before. I am totally committed to being a better man to you and us moving forward together. Are you as well?" If she says yes, then maybe you say "help me to understand how that is possible while you are still engaging with OM?"
And, forget about whether they ML or not, it doesn't matter. My W was obsessed with OM (in here case more of a fantasy, but she did attempt at least to take it further). A close friend of mine had an EA 6 or 7 years ago and he said that it would have been easier to end it had it been about ML. They never did, but the fantasy that he had from their friendship was more powerful than any reality. He ended all contact and went to MC with his W and he said it took a good year for the fantasy to shake loose in his head. The good news is they have a great, and much more honest marriage now than before. It can be done but it takes a mutual commitment and a lot of hard work.
CB
Me; 42, W; 43 M; 16 yrs S12, D9
3/13 - "I want to move to XYZ City (four hours away) and it might be without you, not sure" 5/13 - "Not sure I meant that"