As much as you would like to see the good in what she was doing.....I think it was all based around OM being available or not--and using that other night to throw you "off".....and it did. You didn't even have to go meet her and it threw you off. I can certainly understand why it did, but this is the deal here......you must stop trying to figure her out. You confronted her about the OM and now she will either get more sneaky or she will openly have an A while staying with you. I got the impression from your past post that she didn't have much to say about it when you brought the subject up. She may have wanted to tell you over drinks--that she had found an apartment. But who knows.
GAL is def. what you are to do, but I don't think you get the entire concept yet. When she asked you to meet her in the city for drinks...you jumped on it! Why didn't you tell her that she should have asked about it earlier b/c now you've made plans for the evening. You see, you "always" have some kind of plan in your back pocket to use at times like this. You have to stop being available to her. Stop playing second fiddle to OM.
Okay, you said you have proof of her A. Don't tell her how you got it. The fact that she knows that you know.......or does she know that? You said you confronted her.....does that mean you told her that you knew about OM and that you had proof.....or did you ask her about OM?
Men have to learn how to set boundaries when their W is in an A or she will not respect her H. If the two of you stay under the same roof-- then she should not be in an A b/c that is the most disrespectful act a W can do to her H.
Have you thought of how you will word this to her? (I'm talking about the boundaries.) Don't wait to see what happens and then try to think of the right words. Don't wait to see how she sets the tone for the evening and just hope for the best. Be prepared. Have a plan.
It is not about what you feel should work in your M. It is about doing the work that gets the right results. Do what works!