Full transparency, not staged transparency.
Take his phone and his hard drive to be data mined. Even suggesting it will be telling...if he blanches, well, there's your answer.
Data mining will give you all the erased history, texts, photos and emails.
Also put a VAR in his car. Of course, don't tell him, just put it in there with some velcro tape. He has some kind of secret phone that he is using to yuck it up with her. This is what I mean by 'staged transparency'.
IC for him to figure out why he is such a douche'. And you must have access to the IC to check his stories, he is a known liar.
(Mr. Happy is in IC and I have full access)
Accountability as to his whereabouts like a GPS finder.
Access to all CC statements, bank statements and phone records. This way you can check his stories about his whereabouts. And they must go back farther that he states the beginning of the A.
There is a poster on these boards that is a forensic accountant. She was blindsighted by the A, but honey...with scalpel like presicion she parsed apart his doings and laid it out for him to see...HA!
Like a client once said, the internet is like toxic waste, nothing ever goes away!
His cyber history is available, ask him plainly for all passwords and all accounts.
Just because he gives them to you does not mean everything goes back to normal, there will never be a normal, ever again. The 'new normal' will come into play. And both of you will have to agree on new terms for this to work.
Do this in a calm manner. Business like. And the terms are NOT NEGOTIABLE. A OR B! Either he succumbs to your terms or all bets are off.
These are not frivolous requests. You need to know what is happening in your own life. For your sanity's sake. This is the only way for him to gain your trust and to feel safe.
And as far as 'his privacy' is concerned, he forfeited that concept when he abused 'his privacy' by being married and a having a 'girlfriend'!
Good Luck and HUGS sweet Lackingcourage!