I think you are underselling how awful she was in that situation, she knowingly went behind the back of her best friend who she knows has unresolved feelings for this boy...
I think people making light of that aren't placing themselves in the characters shoes, if you had a friend that fully aware that you have a huge crush on a certain girl, made moves on her behind your back it wouldn't feel so good...
And her justification is that she was just "too involved", again much like you don't buy into what Erynn says, i don't buy into anything she's saying...
You became too involved in 2 hours? Upon seeing this guy that kissed you once, 5 years ago and you never saw again? A kid you barely knew?
And then there's the hypocrisy... if you tell her what she's done to Layla(pizza place scene), she gets on her high horse and judges you.
Plus other similar scenes where she gets to scold the MC for similar things to what she's done and it doesn't even occur to her.
From the POV of my initial plathrough of the game, i was banging Alexis because she (suppousedly)was offering no strings attached pussy, passing the time until someone more interesting comes along(but with potential to grow), she immediately started to show her clingyness which doesn't win you points but ok... but when it's revealed that she's hurting someone that i have a much bigger connection and much stronger feelings for, that's it... no coming back from that.
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.