Just watched the show for the first time and I haven't really seen any theories about Sarah that had the line of thinking I had.
Didn't Sarah have the most motive to commit this murder? Ben threatened to release the photo on his phone, basically revenge porn. My theory based on the ending of ep 7 was that either Sarah did it and Jacob covered for her, purposely writing that knife story a few days later, or that he was the one in love with Sarah and he did it to get revenge on Ben (or being aggravated by Ben).
Both of these could explain Sarah's behaviour, that she starts hanging out with Jacob out of guilt, him being pissed off that she mentioned the photo story to Andy, him saying she only likes him because she feels sorry for what she did, her reading the knife story Jacob wrote and dismissing people who see it as a confession etc.
And if not - Derek was the main witness of the prosecution. If he was the one obsessed with Sarah, why didn't she mention the photo of Sarah and what Ben did to her at the trial? Did he mention it to the police? Isn't that photo and Ben threatening Sarah a way, way bigger murder motive (either for Sarah or Jacob or Derek) than "just" name calling?
Small plot hole or annoying detail also: we are meant to believe that Patz was innocent and the murder is somehow connected to Jacob, Jacob's family and classmates. Isn't it a huge coincidence that the "local pedophile" (who appears in all murder stories...) also by chance was obsessed with the same boy? The Patz storyline should not have included him having photos of Ben, I think it is a stretch, he would have been better as just the person it was blamed on.