Whether he is a walk away or a MLCer, you would still treat the situations pretty much the same.
Now, I want you to put that stinking thinking aside. Put that glass of Kool Aid down. You are buying into all of that stuff he's said. Trust me, if he were that unhappy, you would have known it a very long time ago. They start to mull things over and the depression starts about 18-24 months prior to the bomb drop.
Honey, we all have made mistakes and no marriage is perfect...but when they start saying that they've been unhappy for one, ten or 20 years...I just don't buy into that BS.
Sure he appears happy w/the OW. She's new, doesn't have a history w/him and she most like tells him what he wants to hear. OW do this stuff to get the man and then use all types of tactics to keep them. Some day, that newness is going to wear off and that's when the warts and wrinkles are going to show. It's a fantasy world that they are living in. Trust me, responsibilities and accountabilities will come into play and he's going to discover that it's not always greener on the other side of the fence.
Keep posting. We are here for you.
Sit quietly, the answers will reveal themselves when you least expect them to. The past is gone, the present is a gift and you need to focus on today, allow the future to reveal itself when it is ready.