These and many more was mine reasoning too but I did put myself in Alexis' shoes, MC's shoes and Erynn's shoes and thought what would I do? I could not find a solution that does not make me bad.
I think you are downplaying MC's meaning to her. We also know MC is exactly her type also. So even if MC was a stranger she would fell hard for MC and he was not. MC clearly meant something more to her since she even kept the top from first kiss. He was her first kiss, even though it was childish and most likely her first crush.
She really tried to stay clear of MC also and gave in only after their 3rd random meeting. Everything MC did, somehow effected her deeply. Inner monologue about MC saying hi to her was an example for it. She did not choose to feel like that way, she even tried to deny it. I think that would be my first reaction also.
What she did to MC was terrible even inexcusable but yet understandable. She thought MC cute even before she knew who she was. At that moment she did not know why MC left then when MC revealed the reason, she noticed she was the reason MC left along with Ethan and Erynn. At this point MC hates Alexis as much as he hates Ethan. There was no way, she could have explained that. So she kept it to herself. Even MC admitted he would have acted differently if she told on the plane. Then their story goes like this for each of their encounters. Maybe she could have explained it after the concert but as she said she wanted to tell Erynn first.
What she did to Erynn was bad too but as far as we know, the moment she noticed she fell for MC she decided to tell Erynn. Erynn was the one who cancelled their talk, then immediately after thing blown up with MC. She could not know what to do, how to say. Like I said what she did was terrible but not without a reason.
From Erynn's shoes feeling betrayed makes sense but Erynn should have known that she has no claim on MC. Initial shock was on point, then after that forgiving Alexis made sense also. There were a few things in between but this is not a post about Erynn.
From MC's point, this is where it gets tricky. Irl, I would not give a fuck about Alexis' reasoning so her path would end for me there but if somehow, by some miracle, I'd hear her reasoning then I'd would forgive her and ask her forgiveness for being too mean to her.
I also don't like how clingy she is, also she tries to move forward at every opportunity that is a big turn off for me. Like kill/fuck/marry game, if you choose marry Alexis option she immediately starts making wedding plans and not even joking about it, she is dead serious. Even though MC tries to play it off, it was a wtf moment for me. A few other stuff too, no need to make this post longer. So I like her, I'll play her path but she is not on the top list though does not share hate list either.
I don't think i am downplaying, it's an established fact that they barely ever talked, they had no friendship and she didn't have a crush on him or anything, their relationship was so small that the MC has no idea who she was...
Alexis doesn't recognize him immediately but remembers after he mentions Erynn, the only thing you can sell me is that she has fond memories of that day, looks back at it like a cute story... but not much more than that.
And i can understand her getting the hots for the adult MC, but personally i think a friendship should be worth more than that.
She did a piss poor job trying to "stay away", let's recap, she was the one who invited him into the bathroom, (second meeting)she asked him to stay until she leaves the bar and invited him into her house and came back with a lingerie and (third meeting)she picked up his phone from Zach and bootycalled him the next day... that's as bad a job of "staying away" as it gets, she's the initiator every time, her words don't match her actions in the slightest.
In regards to Erynn i think you have that backwards, again established fact Alexis had multiple chances to tell her, didn't say anything and Alexis was the one that cancelled the concert with Erynn...
Alexis was the one scared of this conversation because she knew she had betrayed Erynn's trust, and that's why she kept postponing it.
Erynn didn't even know there was a conversation to be had... she talked to Alexis about being happy the MC being back in town to which Alexis must've answered "that's great" and witholding the fact that she was banging him while this conversation was going on.
Erynn reacted with fury innitially but has been tremedously understanding, supportive even, since that(despite the fact that it's hurting her inside).
It's no secret that you can be in a relationship with Alexis while still having Erynn in love with you... and she does a much better job keeping it in her pants than Alexis did(same thing applies to Iris).
We differ in the sense that i heard her reasoning and thought it was pure bullshit, can't say a single thing that landed with me "i was scared to tell you", "i got involved" (as i said before way too quickly for me to buy it) and "i tried to stay away"... that's a laugh, try harder...
Also, when i play a game like this i try to get myself in the headspace of the protagonist, one of the premises of this game is that the MC has a long, somewhat troubled, history with Erynn, and despite some wounds he still cares about this girl, that makes me automatically protective of her and caring about her well being, when Alexis actions harm this relationship it's an automatic disqualification.
I do agree with your general reasons why she's just not for me even if i were to forgive her initial actions.