Yeah, the Erynn angle is obvious (I just checked she says she needs to pay for 'the apartment'.
). Regarding the Layla bit, I am a bit skeptical. There has to be more than one school in the city. There can have been more than one hot new guy. So, I don't see the point of her trying to hide the MC. Did she have sufficient context to infer that Layla's hot, new guy is also her hot, new/old guy? (not rhetorical. Genuinely asking since you are the proofreader) Unless Layla has had a crush on MC since before he left. But in that case how does MC not leaving simplify matters?
Well, I see the whole Alexis and Layla situation as, if they are sisters and have a close relationship, that Layla would have told Alexis about the new guy at school. She would have especially told her sister about him when they went out to a club with Elena and Zach. A lot of this would occur "off screen", an example would be when Alexis picked Layla and Elena up at the train station. We didn't see that, but if Alexis is Layla's older sister, then she is the one who did that.
Part of the confusion is the fact that Madison Springs is the name of the large city AND the small suburb... So the city they travel to in order to have a night out has the same name as the place where they live. The large city might have multiple schools but the suburb could have only one. SO this explains why Alexis was surprised to see the MC at the bar (I love it when people outside of the US have Americans drinking in bars at 18 years old, I don't think that is legal anywhere in the States) which is located in the suburbs. She could have expected to never see the MC again based on the fact that they flew into the large city of Madison Springs.
However, once Alexis sees the MC with Zach (the guy who has been irritating her little sister for years) and that the MC is in the small suburb of Madison Springs then it makes sense for her to know that the MC goes to the same school as Layla and that the new guy that Layla is crushing on is the guy that Alexis is "dating".
Layla didn't know the MC prior to his leaving... That we know of, though as she was the younger sister of Alexis (the other girl in the flashback) there is a possibility that Layla saw the MC prior to his leaving but doesn't remember it.
I believe that Alexis is worried because she knows how much the MC means to Erynn, Alexis knew that she was fucking the guy that Erynn was waiting for all this time, and Alexis kept this information from both of them. Alexis could be thinking that if the MC had never left then this drama may have never happened. Either because the MC would have always been with Erynn or because Alexis and the MC could have been hooking up without subterfuge on anyone's part. Remember that when people feel guilty about something their internal monologue doesn't always make sense... Guilt =/= logical...
Anyway, now that I've written a book I hope that is a little bit clearer...