Some more thoughts...
As mentioned, knowing whether or not Jean or the other Jean and John had a car would be key to ruling them out or not since her body was transported in a vehicle. I know a lot of hippies traveled in VW vans, as did Bruce Davis himself. On the other hand, a lot hitchhiked too, especially if they didn't come from a middle-class family. We don't know how Jean and John got from Canada to LA and how they got around town once there. If Jean did kill her, he'd have a lot of explaining to do with the other John as to why his van looks like a horror scene inside. Kind of hard to hide from others. This is why I'm suspecting the murderer owned their own car and did not share it with anyone else.
Communal living among traveling hippies was the norm back then. Very few lived alone unless they were planning on staying in the city long term and secured an established source of income. If we are to believe Jean that Reet did move out of his place, she very likely must have moved in with her murderer, an unknown person she met while in LA. I ruled out Manson earlier in my theory, but I thought back to her postcard saying she went to the beach a lot, and my next thought was....John Haught's death was in Mark Ross' bungalow across from the beach a week before her death....
One theory for ruling out Jean....If you are Jean and your argument with Reet came to a deadly crescendo, is the first place you think of as a recent LA transplant is Mulholland Drive? Seven miles from your apartment through winding hills in the middle of the night without GPS as you're panicking looking for the best spot. If he was a first time murderer, I think he would try to look for an alley, a dumpster, or the first steep hill if he is entertaining the Mulholland idea. I know that area well and the ravine where she was dumped is deep into the hills, suggesting the killer may have even buried here before, like Marina Habe's body, not far away. Unless Jean has lived in LA previously and knows the lay of the land
and has followed the news that Mulholland is a common dumping ground,
and has a car, I kind of doubt he did it. The Fifth Estate documentary could have asked Greta and Paul about Jean's temper. I forget if Greta ran into the tall Jean or the shorter John, but I remember one of them stayed behind in LA. The shorter John with the Beatles-haircut is an even more mysterious figure that I want to know more about.
As for her place of murder, I'm starting to think she was killed inside a vehicle in a parking lot, as I suspect Habe and Gaul/Sharp were too. She could have also been killed inside a vehicle while in the Mulholland area, if her killer suggested they go stargazing. If she was killed in an apartment, it would cause too much noise unless she was beaten unconscious or drugged first, which the autopsy did not find. Plus, you must have the gall to be dragging a dead body out of your apartment at night without being seen. If he lived in a house, it could be easier to have done it there.
As for her family keeping quiet, besides the fact that they were a private family that came from a prestigious background and didn't want to tarnish their reputation, I'm thinking if they knew Reet had a drug or behavioral problem and didn't want it made public when questioned by authorities. In the Manson Speaks documentary, one of her friend claims she did methadone, an opioid treatment. I also read that cafe where she hung out with Jean supposedly had some drug dealing activity going on at the time. We all get the sense her family is not telling us everything and it may be because they know her disappearance had something with her lifestyle getting involved with the wrong people and activities, thus they never actively pursued her disappearance sooner and assumed the worst. Now, I know there were no drugs or alcohol found during her autopsy, but it's possible it may have been cleared her system by the time of her death if she was a recreational user. Again, just speculation.
From a timing perspective, we know that postcard was mailed on Oct. 31st. If we are to believe that Paramount apartment was rented by Jean and John where Reet stayed at, something must have escalated very quickly for her to move out, as that postcard sounded like she was very settled down. I kind of doubt she moved out into her own apartment given her lack of job and income. But then, we don't know how much money she brought with her.
After a lot of thoughts on this, I really do believe she was murdered by an unknown person she just met out here.
What truly makes me sad in all this is that Reet didn't even get to enjoy the freedom of living on her own for more than two months. Didn't even get to live to see the '70s.