Bart, I've been where you are so I understand your frustration. It's hard to set the boundary of 'work on the R or I file' if you're not ready. It took me mos of being in the dark about W's A (or maybe just keeping my eyes shut) until I was ready to lay down that option.
I can say that 6 mos ago, when I first discovered evidence that it was more than a friendship, I told W that as long as she was in a R with OM, I didn't think she should stay at the house and as long as she was friends with him, we wouldn't be friends. That did lead to her moving out, but I also think the relationship with OM heated up. I also did a lot of waffling on being friends with W too, which probably didn't help.
I wish I would have taken a stand earlier on, but I wasn't 'ready'. Keep GAL-ing and spend time evaluating your options and what you can/can't live with. I truly believe the earlier you take a stand, the better chance you have of saving your M.