I personally don't think Jay had anything to do with Hae's murder or had any inside info about her demise whatsoever. I think he let the cops bully him into making up a story based around bits of information that he was fed without even realizing it. He was dealing drugs and even talked about how the cops harassed him all the time in his interview with The Intercept. IMO, he just told the cops what they wanted to hear so they would finally leave him alone.
While I have no idea what happened to Hae that day, I am curious about the discrepancy with Don's time card at LensCrafters. Plus, I always thought it was weird how everyone was so hung up about Adnan not calling her that night or over the next few days but never questioned why Don didn't. He was supposed to see Hae that night IIRC, but he never paged her when she didn't show up (which wasn't typical behavior for her)? Or when he hadn't heard from her in days? And then there's the thing about him not calling the detective back until 1 AM. I'm not saying I think he's guilty of anything, I'm just curious why his alibi wasn't investigated more aggressively.
But....he was a penny ante dealer, and in the suburbs, not in Baltimore City. IMO his stated fear about the cops busting him for drugs is completely bogus, just a handy dandy, too tidy "explanation" for his stream of lies and evasions.
IMO, there is little doubt Jay was directly involved. After listening to the Serial podcast when it first aired, I thought Adnan was more likely guilty than not. After just recently reading all the original docs via Undisclosed, I'm leaning towards Jay and just Jay.