Another well-detailed analysis, but there are just a couple of points that I would question.
Maya and Josy actually got together the previous summer because Josy talks about graduating from high school "next year" and that she'll probably get a part-time job before it because she enjoyed having a job selling strawberries before the camping trip. So Josy and Maya have actually been together for about a year, or at least a full school year.
As I said, it takes place sometime before the summer. I tried to figure out exactly when that scene took place, but there's no clear indication exactly when. It could take place at the end of the previous summer (since Josy talks about her job selling strawberries in the past tense), or in the fall, or in the spring; I think we can assume it does not occur in winter.
One thing that initially threw me is that Josy is already "hoping" she'll get into B&R whereas Maya just says she and Derek are going there. That made it sound like it was the spring, so Maya and Derek were already accepted and Josy was on the wait list. Going over it again, I think it's more an indication that Josy already has doubts about her ability to get into college. It's hard to say for certain, though, because B&R's acceptance schedule is just flat out weird.
I believe those mean-spirited texts were just being sent after Josy and the MC's 2nd date since Josy had her phone taken from her, not that Maya was sending them all summer. The last time Maya likely heard from Josy was when she told her she was planning to move away and then all of a sudden Josy becomes incommunicado, leaving Maya feeling very hurt, confused, and wanting answers, but not hearing back from Josy makes her text a lot of mean comments due to her emotional state.
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The texts in question were definitely sent while Josy is 'grounded,' I just point it out because the trend may have started before then.
Another thing that struck me on examination was the way Josy phrased "Relationships when you can't be with each other every week suck, by the way." She specifies seeing each other every week, not every day or some more nebulous measure. She repeats the "every week" measure in her later phone call to the MC.
I may be reading too much into it, but it leads me to believe Maya's dad tried to break them up during the school year. They weren't able to act as a couple, but they were probably able to find times in their schedule when they could still at least talk to each other. This is when they dreamed about going to B&R together and finally getting a chance at a real relationship again, even if it had to be in secret.
When the school year ended, they no longer had that routine and that's when things started to fall apart. Maya's dad couldn't kick Josy out of Maya's school, but he could certainly keep her out of the house and monitor Maya's trips to friends. He likely also restricted her phone/internet access, so even talking to Josy was hard. Given that Maya's dad expects Derek to keep an eye on her for him, it's possible a lot of the communication they did have went through Derek.
I think, if we break it down into a timeline of events, it might look this:
Good timeline. The only things I'd quibble about are when Maya's dad got involved, and when Josy told Maya she was running away.
It's tempting to think the call Maya got on Sunday was from Josy, but I don't think it fits the timeline. We know Josy's parents got home late Saturday night, because they will interrupt the sex scene if the MC stays with Josy. It's hard to see how they would wait for the MC to return to B&R and chat with Maya before confiscating Josy's phone.
I also don't believe she would call Maya before the MC at that point if she did have her phone. When she recovers her phone on Wednesday morning, she definitely tells the MC about her acceptance instead of Maya. If Josy did call Maya after the second date, I think she would have definitively broken up with Maya. She'd already quit her job and does not yet know she's been accepted at B&R. While she might not have argued with her dad yet, it's obviously going to happen soon. If Josy proposes running away at that point, it can only mean she's giving up on her old life, Maya included; there's nowhere else for her to go.
I think it's more likely she called Maya just before she quit the minimart. It was her last effort to salvage the relationship, when the idea to run away could still be speculative, something Josy didn't want to do was afraid she might have to. When Maya couldn't understand that, that's when Josy impulsively decided to quit, give up on Maya, and resolve to sleep with the MC; it just turned out to be harder to go through with when the time came because she never stopped loving Maya.
Sadly, I can't see much concrete evidence either way. It's possible (even likely) Josy had a way to contact Maya without her phone and simply didn't know the MCs phone number from memory. Maya seems a lot more open to a relationship with the MC on Tuesday than she does on Friday night, but we can't tell if it's because Josy's last call almost ended the relationship, or if it's the result of Josy (apparently) ignoring Maya's increasing bitter texts for several days.