I am so very sorry that you are having a difficult time at the moment w/your h being MIA. You need to find a way to understand that when they are in MLC some will completely disappear and will go radio silent w/the spouse. You are looked upon as being the authority figure that they are rebelling against, therefore, they do not want anything to do with us. We are the first that they cut out of their lives and if they should be lucky enough to return to us, we will be the last that they reconnect with. You have to remember, it's not you...it's all about him.
You don't know what he's doing, so you need to put the "assume" away for now. He's getting his attention "kibbles" from the OW at the moment and she may be the type to stroke his ego and make him feel good about what he's doing with her and for her and the kids. Not all ow are crazy or personality disordered. Some actually have very good jobs and are intelligent, then there are others who aren't all that great. You do not know what he's told her about his situation and let's face it, they become very good liars and painting the "woe is me" picture for the attention and sympathy of the other person (op).
The ow is nothing more than a band aid to what ails him at the moment. Being w/her is a distraction so that he doesn't have to think about what he needs to do to figure himself out. She is nothing more than a distraction and not special at all. She doesn't know him the way you do, nor has she shared a lot of those years w/him that you have good memories of. Always remember...you are the prize and can honestly hold your head up and look people in the eye and face yourself in the mirror each day because you didn't break your marriage vows.
For now, you need to keep the focus on you and the fight you have going on w/getting your good health back. I know it's difficult and you are sitting there wondering about him and that's okay...we all do it...but your focus, strength, and determination right now need to be on 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.