AnotherStander, would you maybe explain a bit more on " but that all changes in a S." ?
What I meant is if a spouse has gotten to the point of actually walking out the door and not just threatening it, then they're no longer interested in working on the M. Once they walk out the door their focus is on themselves, finding the "happiness" they think has been "robbed" from them by the LBS. In time they may be interested in reconciliation, but not right after they walk.
So trying to set boundaries is difficult because they'll just see it as pressure and they'll likely think "what do I care about their rules, I'm on my own now!"
And if they walk, about the only boundary enforcement you can set is denying them of you. But if they walked, they're probably wanting to get away from you anyway. So if you say "no contact with OP, if I find out you break this rule then I will go NC on you." They're likely to think "what do I care, I left to get away from you anyway!"