I guess my greatest immediate fear is that if I leave now I'm just sealing the deal with OM. She'll have that last little excuse she needs to go ahead and cross that final line.
Uh...no no. That's the problem right there. You are acting like you're responsible for W's choices. How wrong headed is that? Oh and affairs SELDOM do last.
FWIW, many of us continue DBing even if the A is a PA and ongoing for months.
It really comes down to FEAR. Fear of losing W??! Well, you've already lost her.
Trust me. You're not losing W by moving out. Let's suppose that W does take the A to the next level, then what are you going to do? Be the henpecked H and just take it lying down because you are too afraid of reclaiming your true self.
C'mon...take a look around the DB forums. 99.9% of DBers have spouses who are deeply involved in a PA. And come back from that so-called 'red line.'
In the final analysis, making the choice to engage in a A is all on W. (doesn't really matter if it's EA or PA--semantics)