Lacey not contacting MC through her senior year makes sense. She says multiple times that she wouldn't be able to help but run back to him if they talked. Was it fair to MC? No, but Lacey's reasoning was consistent there. What I'm more curious about is why didn't she talk to MC after she got her job while MC was finishing school. I personally think it's because she doesn't feel worthy of MC's love and procrastinated until she knew MC had his own place, but I would like that question directly answered in a future act
For the 3 weeks of partying thing and why I think Lacey didn't do anything more than teasing/flirting + some groping.
1.) After those 3 weeks, Jared says something along the lines of "You haven't won. this isn't over. I will fuck your wife and make her my side piece" I don't think that's something someone would say if they were successful.
2.) It's shown that Jared would often bring girls into the Supply Closet, but the groping that MC saw was the worst thing that happened between Lacey and Jared. If Lacey was sucking/fucking Jared, then I think it would have happened in the supply closet too. NOTE: The guy that got the security footage was doing it for Veronica, so it's possible he purposefully left some out with her directions
3.) If they fucked, I'm absolutely sure he'd have evidence of it since he is all about blackmail
For the 3 weeks of partying and what I think happened
Lacey is all but confirmed to have been dancing (probably provocatively). However, she can also be heard telling people to "stop it". It doesn't sound like she knows it's MC on the phone either because she says "who's there" after telling someone to "stop it". I do think groping was happening, but I also think she was telling people to stop. This lines up with her later statements about her enjoying flirting but "If I think they're getting any kind of hope of something more than a chat, then I shut it down"
To summarize, I think Lacey enjoyed teasing/dancing and getting attention/validation, but when people tried to push for more, she shut it down