The fact that you were drunk won't lessen the amount of hurt the truth will cause her. It's better to not say anything if it truly won't happen again. It's something you'll just have to live with and try to put behind you. I had a similar situation happen to me where I was drunk and almost ended up doing something with my bf's friend (we were seperated at the time). We didn't do anything but he found out about the situation and thought we did. He felt betrayed by his friend and felt like he couldn't trust me anymore. It really hurt him. That was years ago and he still brings it up and I think he still has some resentment for me over it.

As for the guy, the fact that he did everything but sex doesn't mean he didn't cheat. He totally did and he's just trying to justify it to himself. Hopefully this was a one time thing for him too because if it wasn't then it'd be better for them to breakup anyway.

As far as booze goes, it can be pretty hard to quit. When I drink I can't stop after just one so I know its either all or nothing. I tried moderation for a while but ended up right back where I was in the beginning. Having people you can talk to about it like AA or r/stopdrinking can help encourage defeating the beast lol. Good luck.