With the information from subie that her her boyfriend (?) had borrowed her car and not returned it previously, I'm wondering if she had it at all that night. There's been several suggestions as to why she would be driving her mom's car, but it being driven by someone else seems the least speculative, as we know it had happened before. Also per subie, she'd had belongings in her car for some time, not necessarily placed there that day. If that guy or someone had it, they may have been afraid to come forward (with or without good reason) and left it near the studio, either because that's where it already was or as a dump site.
That said, it's harder for me to figure out where Jenna was when she went missing. The easiest explanation would be that something happened at her parents' home, then Jenna and a potential perpetrator left, with her purse and phones, in Jenna's car, or Jenna and a potential perpetrator were somewhere, with Jenna's car/purse/phones, and something happened there. Based on available phone data, it sounds like either she was staying put from ~1130 on, or her PCS phone (this was the one with Timeline on?) was no longer recording her location (low power mode, dead). Do we know which phone her SC friend had contact with her on? I assumed the T Mobile.
As far as "lying down", I could see it meaning that she was upset and going to get in bed but not necessarily sleep. She might've been upset enough to go find this guy somewhere after that, even, but I don't understand why it wouldn't show up on her timeline. I would think she would charge her phone at home or in her car.
And finally we're at the point of wondering why exactly it was that she went missing. The car was found in an area of some criminal activity, if you will, but nothing that fits the m.o. A drive by shooting, mugging--these are very different from what seems to have happened to Jenna. In those cases, we would know exactly where she was, and finding her would've been the first clue, and not the last. The first possibility is that she decided to leave on her own--with no obvious vehicle, money, phones, I can't see this. I know others have done this, but this level of planning and deception seems beyond her resources. The second is that she was injured. If accidentally, then during an argument, maybe, or a drug overdose. Sorry for the degree of speculation, but finding a viable motive/scenario would be helpful with establishing a crime scene and narrowing the suspects.