Anyway, I'm pretty sure Adnan didn't do it. A lot has been made of the Nisha call, but I think there's a simple explanation even if Jay accidentally dialed her.
The call lasted 1:30, which is pretty long considering she didn't have voicemail and Jay didn't talk to her. But since she thought the call was from Adnan, she could have been listening to Jay's conversation trying to hear what they were talking about.
I do this all the time when someone accidentally calls me. Not if it's just the sound of their phone in their bag, but if they're having a conversation I almost always listen for a while. Usually I spend a while listening, and a while shouting hoping the person will hear me through the receiver. It may not be super polite, but since they're the person that accidentally called I don't really feel badly about it. Besides, it's usually someone who's close to me, if it's a casual acquaintance I hang up immediately.
The point is, if I do it, I'll bet other people do it, too. So maybe Nisha listened for a minute, didn't hear Adnan, then hung up the phone. Six weeks later it wouldn't be a really memorable thing, especially considering how most of those kids sound like they were constantly getting high.
I still don't have any theories about who did it, except that I believe that Jay really was involved as an accessory after the fact, and that he was scared of whoever committed the murder. Maybe a friend of Don's?