Sounds too familiar. My H is seeing OW that he met on Facebook while putting together a HS reunion. They went to HS together and he seems happy as a clam being the person bringing all of his friends from the 70s back together. He is definitely still in the fog and there's nothing that I can do. So, he's not seeing a younger OW, but someone that brings him back to a time when he was a teenager. I agree with mkultra - too easy these days to reconnect with your past.
You are brave to have gone dark. It's a tough thing to do and something that I haven't been able to completely achieve. Does your H ever mention D? Have you decided what you want? You said that the door is still open a crack, but I was wondering if you are still trying to salvage your marriage or if you have gotten to a place where you can see moving on from it? I can't even imagine ever getting to that place, but my S is still very new. We're all in a very painful place. Glad to have this board.