On some level you know what you need to do...that is why you are having the thoughts. She be able to get another man just as you will be able to get another woman...that means nothing. Quality vs quantity! You are quality and the father of her daughter and she knows it. Stats prove that these kind of realtionships do not last
You have to do something and it should be the shock and awe concept...she is riding the fence and this will continue as long as you let it. This is a script that has been written a thousand times. Quietly start paying all your bills off that you can, cancel any joint credit cards, try to get ahead on your house payment, then go file.
Please remember that you are either married or you are not. Which means your wife becomes a wife and mother again, or she doesnt. And a woman that would do these things to her husband and daughter should not be your friend.
Cordial in dealing with your daughter? Yes. A friend? No. It should be limited to generic talk about your daughter and what is best for her. Matter of fact stuff. And then when she wants to have friendly talk, you just leave the room or get off the phone.
YOU are the only sane parent your daughter has. Do not let her have much influence over your daughter, if you can have a say in that. And I believe you can, by getting full custody NOW! Dont wait until she returns. File now, have all your ducks in a row.
Don't keep dragging this out...with her leaving your daughter with you she has set a precedence that will be hard to explain.
The longer you allow this to continue the harder it will be later...try to get proof although you know she is in an affair. It is not over.
Good luck on Thursday...you deserve it! Praying for you and yours...you do the same.