I am starting to wonder if there is something entirely different going on here given LEs seeming lack of interest in the story.
Here is a theory. Subie and family, I apologize in advance if I am way off base. Just trying to think out of the box.
Let's assume Jenna was involved in sex trafficking and she was picked up that night in a sting.
This would explain the "found" by another jurisdiction (say Atlanta) very early on.
Let's assume that for a lighter sentence of some sort, Jenna agreed to go undercover for this jurisdiction to ferret out the main players or has been put in hiding in order to identify them at a later stage.
This would explain phones going off the grid. And the retracted "found" (how do you misidentify someone!!!). This might also explain the backpage picture which looked a whole lot like her. And Ex's sex worker comments. And perhaps roommate is involved in the sex trafficking operation which is why he was reluctant to let PI and family into the house.
Let's assume that LE wants her to stay "missing" because it helps them to catch the traffickers.
This would explain lack of household prints until family pushed for it, not reporting the finding of the car, just general disinterest in this case by Dekalb PD without the family pushing for it.
Let's assume the police finally told the parents what was going on because they were suffering so much.
That would explain why PI no longer on the case, family not raising a fit with the media, fewer flyers around, lack of searches and door to door knocks and suspect interviews, perfunctory answering of questions on this site, etc
Let's hope this is the case and she is alive and well and helping to break up a major sex trafficking operation in Atlanta