I agree that you need to put the focus on yourself and your child. If an EA/PA is going on there really isnt much you can do about it and IMO anything you do try and do to stop it will only push your W right in to the arms of the OM.
Your W sounds very much like my H - he expected old feelings to return in a matter of weeks in some sort of magic way and it just doesnt work that way. The rebuilding process is slow and nothing will change overnight.
I am not a big fan of ultimatums because rarely do they go the way you want them to and if you do issue one, be very prepared to get the answer you DONT want to hear.
Both parties need to be on board to rebuild a marriage and if your W does return to counseling or attend a weekend seminar with you it has to be out of desire, not guilt or obligation. And truthfully, if there is another person involved nothing can be rebuilt until that person is totally removed.
That being said there is nothing wrong with your stance of an affair of any sort being a dealbreaker. There is nothing wrong with saying you are done and ready to proceed to the next step of ending your M in a legal way. Just be sure if you say that you mean it because once its out there you cant take it back.
You sound angry, hurt and very emotionally which is okay. Feel what you have to feel but IMO I would wait at least a week before you make any further decisions or deliver any ultimatums. If you still feel that way in a week, well, then perhaps your feelings werent being fueled so much by emotion but with actual desire.
Living in limbo can be a real kicker and sometimes I think we just want a way out. And there is nothing wrong with that. I just offered you my opinion because in the past I had a terrible habbit of making decisions and reacting on emotion in an immediate fashion and it never turned out good.
The thing is, you cant control what your W does, how she acts or what she thinks. What you *can* control is how you react to it. It took me close to a year to really grasp that concept (yeah, I am slow, lol!).
