Hmm. I went through exactly what you describe, minus the Swinger site membership. You are most definitely NOT over reacting. I was with my ExH for 20 years (married for 17). Around 16 years in, I was told by a friend, that he had tried to sleep with all of my friends. It was devastating, but sadly true (I asked my friends and they all verified). At that time, past events that I swept under the carpet, resurfaced in my mind. Like the time he was staying over at a friends in town, but didn't actually sleep there (slept at a female colleagues instead - all innocent apparently) and I realised the little things he gaslighted me about, were probably not innocent after all.
A few things I can say that might help you....
I stayed a further 4 years (2 kids and scared to leave), and I can tell you, that even though he knew that I knew, what he'd done, it continued, every time he had a drink the same behaviour resurfaced. So, I'd say that your H is unlikely to suddenly stop these ingrained behaviours, especially if you "let him off" this time.
My ExH also told me (years previously) in a row, that he would sleep with other women (I swept that one under the carpet, as a throw away comment in a row....but seriously, now that I look back, that was a stupid under-reaction)
Regards the violence....if you decide to leave him, be prepared for this to escalate. I also had been subjected to a rare push/kick....extremely rare, however, when I was leaving him, he assaulted me twice in a much more serious way. I mean how dare I leave him? Men who are so entitled, cannot conceive that you are leaving them, and that they have lost control.
If you are concerned about finances when splitting, be aware that you are entitled to half of his pension - but you get that in cash. So, once you've sold the house and paid of the mortgage, whatever equity is left, you are entitled to a larger chunk of the cash, if his pension pot is bigger than yours.This was my saving grace. Get a good Sol to draw up a separation agreement. You are also entitled to "economic recompense" (more extra money) if your career took a back seat to care for the DC, and his career was unaffected.
If you want to do more snooping, be aware that even if he has opened tabs in Incognito Mode, he might not realise that the search tab still predicts what sites he wants, going by past searches. So, for eg. he has searched previously for Swingers.Com, if you put an S in the search tab, that will come up as a suggestion. So you'll know he's searched for it. This is how I found out that my ExH had been on Adult Friend Finder, which is a sex hook up site. dirty fucker I literally went through and typed every letter of the alphabet into the search box.
He could also be on that site, or Ashley Maddison or Illicit Encounters.
Fwiw, my Exh went on to have another serious relationship (lived together) and he did the same things behind her back. Leopard/spots, and all that. He's now 50 and all alone. Although he's out most weekends, most likely stalking for women (uugh).
I am happily remarried now and the difference is remarkable. No cheating, no secrets, no sexting other women. Ironically, my DH is ten times better looking than ExH. He thinks that my ExH had "small man syndrome" - always looking for female affirmation, because he was a weedy short man, lol. DH is 6ft 3 and needs no such affirmation. Just a theory of course...
You could do so much better than this. You deserve to be with someone who you can trust. I'm so sorry you are going through this. 