What are your plans under the two paths? If he straightens up, then where do you go? That’s Plan A – a reconciliation of the marriage. If he doesn’t, then what do you do? That’s Plan B – divorce (or whatever you are planning). Play the best of both.
For now, treat him and the M as if Plan A is in effect. Give the benefit of the doubt. But tell him that you are also going ahead in planning and preparing for Plan B as a contingency backup, but you “vow” not to go down that road unless he does not keep his “vows.” You will need to lay out exactly what the critical triggers will be to cause you to implement Plan B so he knows exactly what is expected of him and exactly where he "fails." But this way he can bear some of the responsibility for which path you take. Let him control the destiny of the M, not you.
