You are getting some great advice from the people here. All I can tell you is to get on with your life, and, as long as OW is in the picture, the marriage is not being worked on, although you both are using a MC.
The rollercoaster can be ridden for as long as you choose to ride; and you will have to realize you cannot fix him, or his issues, and just get on with your life; all the while leaving a door open for a period of time for him.
YOU are the only one you can control..and you'll need to deeply question yourself on your motives for continuing your stand for the marriage.
You are the only one who can answer the "what if" questions to your own satisfaction.
These are the only things that come to mind as I read the developments that have come up.
Questions to ask yourself: Why are you still standing when he's bombed you a second time with another woman? What do you think you will gain,(other than continuing self growth) from this standing?
There comes a time when you have to totally let go, and let God..understanding that you've done ALL you can do.
Your mental health can only stand so much before it breaks apart, and you suffer a total nervous breakdown.
You seem to be getting too far sucked into his drama, although you're "dim". You're afraid of making the wrong move, and making a mistake within this. Don't worry about that; HE made a grave mistake, not YOU...and HE is the only one who can fix that, if he will.
You can only take care of YOU, focus on YOU, learn to deal with YOU.
There are NO guarantees in this life, and you have to do what's best for YOU, even if it means letting your husband go completely, moving on, and possibly watching him go totally the other way.
No one KNOWS what will happen whichever way you choose to go, but you will be fine, regardless of what happens.
If he follows, doing all he needs to do to help rebuild the marriage AFTER OW is dumped, that will be great, if not HIS loss, not yours.
Remember, as each person is different, every MLC/Transition is different..what works for one may not work for another. Most of the time it is trial and error for ALL involved.