Having read your elaboration on the matter, I'm still not really shocked about what she's done. I think the issue is more to do with handling yourself when you've had alcohol in her instance, but she's 20 mate. A lot of 20 year old girls behave like that. Not pleasant but predictable.True enough my wife wouldn't show her underwear off in public, but that's because she never gets lashed.If she did I daresay she's have done something like that at some point in her life. If you were not together at the time then as I previously said , not your concern at all (I'm unclear when it all happened).
I have had a few men tell me they'd like to have my wife, that's just a compliment,because I'm secure in the knowledge that they never would.Take it as such, THEY are jealous of YOU, not the other way round.
You are very het up about this and I can't see the jealousiy issues going away, your still angry and feel wronged, and want her to have the blame, so if I were you I'd knock it on the head as it'll end up goping nowhere and have a look for someone maybe older or quieter who doesn't drink and is making it clear they just want to settle down. I can't see a way round this for you mate because you still think the big problem is her behavour ( which fair enough is not great, but she's 20 and you were on a break and at 20 most of us want to enjoy ourselves). She will see your possesiveness, jealousy and snooping as the problem and since you still haven't got over that, then she'll run the proverbial mile.
Really sorry, that that's not the answer I think you wanted, but good luck whatever you decide.