Op I have just read this whole thread and can honestly say, from your last post, he has just done a number on you.
From your very last sentance, look back at it. After all that talking and soul searching, you still don't know if he is going to see her.
My father is a wonderful man, married to mum 50 years, most happy.But about thirty odd years ago he got involved in a 'fatherly' capacity with an employee who had marital problems. Mum was like you, a little unsure but not keen to put her foot down as he was always loyal. But thankful my mum's sister followed him and he was not golfing as he said, he was meeting the woman in a layby.
Nothing happened, but probably because he was busted. He maintains he was being a shoulder to lean on, but he was fooling no one.
I honestly implore you to dig deep here. There is no point in being a cool wife to a cheating husband. There are no kudoos in it. He knew you were uncomfortable before this big chat, he must have.
The fact he is making you decide whether he goes or not is deeply unfair. Unfair and suspicious and manipulative. It means you are the baddie, the jealous wife if you put your foot down. And he could still go and lie.