By all means, draft up a separation agreement. Do it with your L and have your L send it to him. You don't collaborate with your partner on a separation or divorce, you act independently and negotiate through third parties. Trying to collaborate with him or showing him something you've worked out on your own is going to come across as contact-seeking - wanting to shock him out of his reality, get him to start acting different, etc etc. It won't work, and even if it does work, you're still stuck in this game you and he are playing out together at the moment. If you're going to do it, do it through a L and refuse to speak to him about it - he can communicate how sad he is about not being able to have sex with one woman and enjoy the domestic labour and approval of his wife while he does it, how endlessly difficult that is for him, via an email to your L.
The conversation he had with his mistress is not him breaking it off. It's an excuse for more contact where he can talk about how impossible all this is for him and how awful it is, having two women hanging on his every move. The fact he even told you about the way the conversation went is proof of his. He could have texted or emailed her and said he was done, no contact. Instead it's just more of the drama he seems to choose to be embroiled in. It's very teenage behaviour on his part and even without how disgusting it is that he is cheating on you, incredibly unattractive to my mind.
Have you worked out what happens when you say a quiet, respectful and consistent no to him yet? No matter what he says or does next?
