I can accept Number 5 being the killer too.
There's not really much of a backstory about her, sometimes I question why she's still here. Then finally, on episode 9 she tried to leave which she just did so they would not suspect her.
I also read a theory about Chanel being a con artist. Like, when Chad's family never heard of the Oberlin's.
In Number 3's voice, They've had this way too thought out.

That ending would suck, though. I'd rather have it Chanel being adopted and she doesn't know.
Honestly, trying to solve this just makes my head spin! It can totally be anyone and they'll just give excuses and tell the story from flashbacks. I kind of trust the writers on this, because unlike PLL they know who the killer is since before even the show started so I'm pretty sure these plot holes you guys are saying will be explained whoever the killer is. In just 2 weeks girls!!!!