I'm sorry you are in this situation and free advice is sometimes worth what you pay for it, so forgive me if anything I write is ultimately unhelpful.
When my wife started having an affair with her boss a year and a half ago, I had a similar dilemma (she claimed it was only an EA at that point, but I'm pretty sure it was a PA, too). My wife claimed that I lacked ambition because although I was making really good money and was very happy doing what I was doing, it wasn't enough for her own, subjective "corporate" standard, which seemed to have a bottomless expectation of income. She also thought that I was not being "aggressive" enough about "fighting for her." The first part was ridiculous and offensive to me, and it proved to be a difference between our core values; the second part was true, but mostly because I was following the prescribed DB path and she wasn't willing to stop the affair even while we were supposedly trying to work things out for the sake of our marriage and toddler. Thus, I DB'd and put up with this nonsense for half a year until I realized that the passive DB approach was only hurting me, but by then it was too late, and now we are just waiting to finalize our divorce.
This is not to say that Michele's books didn't help me in many other constructive ways, it's just that I think you have to carefully apply the information to your own situation, consider other sources of information, and accept the fact that whatever you do may not in the end change whether your marriage survives or dies.
I have to tell you that I have read many, many of the threads on these boards, and I see precious few posts from anyone who has had much success "riding out" an affair by not establishing reasonable boundaries like the one you mentioned, especially when the walk-away-wife tells the left-behind-husband that he isn't being "aggressive enough," or "man enough," etc., about fighting for her. I think it is hard for wives like this to have any respect for a husband that just passively sits there, day after day, hoping, and "validating," and "[supposedly] getting a life." In many cases, this sort of "look at me, I've changed!" stuff is viewed by the walk-away-wife as too-little-too-late or just plain pathetic--that is, even more repulsive.
And, no offense to anyone out there who is trying to save his or her marriage the best way he or she knows how (because marriage is a beautiful union worth preserving, even if the preservation demands a tremendous amount of pain and patience), but I think that some people get a little too carried away with some of the passive aspects of the DB approach to the point that they neglect to see the forest through the trees. Instead, and even though they are supposedly detaching and getting a life, many apparently turn into DB junkies, endlessly patting themselves on the back for surviving another day as a doormat and writing about every little glimmer of hope they get from a stray smile or comment from a wayward spouse that is regularly crossing town (or international waters) to hop into bed with an extra-marital lover.
In other words, your gut is probably telling you the truth: your wife probably DOES want you to be assertive and stand on your boundary. You don't have to be a jerk about it, just firm and respectful, the way it sounds you have been from the beginning.
In short, I don't think you should withdraw your ultimatum. If that stresses your wife, well, tough. She should be stressed because she is now forcing the issue by wanting to go see your cousin (and your cousin is a jerk for inserting himself into your marriage). I therefore think that you may actually be at some sort of an ultimatum even if you don't think you are.
If you do decide to separate, you (on your own) should immediately consult a family law attorney to find out how to protect yourself against all sorts of unexpected surprises if it turns out that you later get divorced.
