Thanks Chatter. That's exactly what I'll be doing. My non-negotiable boundaries:

I will not be an option

I will not continue a relationship with W for as long as OM is in the picture in ANY way

If and when W wants to come back to our M, I will require proof that OM is out of the picture for good

If and when W wants to come back to our M, there will be new ground rules for open communication, especially when it comes to problems with either of our unhappiness in the M.

I will not debase myself and compromise who I am to try and save our M any longer.

These boundaries are non-negotiable. I will frame my conversation around these, although I know I don't have to explicitly state them.