I just can't be done with the bs anymore, the lies, the second guessing. It's making me ill.
You don't need any more justification than that. At all. Forget evidence and unchecking boxes - your gut is telling you, and your experience for years has shown you, that he's a bit of a snake and you're better off without him.
You could look at this from entirely another angle. He is probably secretly shitting on you. He's definitely treating you badly. He must think himself quite clever really - losing passwords, new accounts? All this secret agent stuff, working his socks off to keep you in the dark. So he must want to stay. He wants this to continue - having his cake and eating it.
How incredibly SATISFYING do you think it would be if you were to get your ducks in a row financially and then just tell him that it's over? Why - because he's deceitful and he's not on your team and you think you'll be happier without him. Simple as that. What does he mean, you can't prove anything? Why does he think you need to, or would be interested in spending your time justifying yourself to him when he treats you the way he does? You're not happy with who he is, that's it - he just has to suck it up and you aren't interested in the details and you don't have to wave proof of dinners for two on his bank statement in his face. You just... don't want to be in a marriage like this any more. Shrug.
That would be quite amusing really and madly satisfying, don't you think? And so much easier and quicker. You KNOW the score. You're currently totally in the mindset of having to have concrete proof of something, but step outside of that and you'll see that a. it changes nothing andb. it makes it all about him and you having to be reactive rather than proactive. Plus, even if he hasn't played away, you will still have that gut feeling. Because he treats you badly, keeps you on the back foot.
I would sort out the finances and make plans to move on. If you can find out what he's up to, I get that it will help (and it's infuriating not to know stuff!). But, I also think that you will gain so much power by simply taking charge and deciding that he's just not good enough for you.