SciDad, I am sorry for your recent turn of events. I understand how painful the betrayal is. If you still want to remain married, or at least increase the odds that you will work it out, I think you are right to keep to the path you have been following.
Stop snooping. Assume she is lying about everything and assume anything is possible with OM. My H now leaves his phone and laptop unattended. I assume he thinks this is reassuring to me, but I know very well that if there was anything incriminating on there he would delete it. My bet is they had a fake account/fake names when they were communicating. Actually I know they did. So I walk right by his phone and I am not even tempted. Actually when I see it lying on the counter (as opposed to being clutched by him 24/7 like he was doing for months) I do not feel relief, I feel revulsion. Now seeing his phone unattended makes me think he is hiding something and leaving the phone out to "throw me off". So there is no relief, there are no reassurances, there is no way to verify if someone wants to truly hide something.
For me it comes down to this. I am committed to this man, to my marriage, to my family. He is struggling right now with his commitment. He can work it out without any ultimatums or "rules" by me, as long as I am safe (not being screamed at, being subjected to STD's, etc) and I can be strong.
I can put my anxiety about the future on hold for a little longer. I can change my mind at any time this is no longer healthy for me. (coming to that point soon.) So stay the course, check your plan, focus on your own goals, do not snoop. Time will reveal all.
Or, decide that this is it. Keep in mind that she is still struggling with her commitment and you have no control over that. Can you live with the outcome if she decides she will continue seeing OM, or if she takes it deeper undercover?