@MassGuy agree with you re timing of arrest (although much less familiar with US police & legal procedures).
In Australia, police usually know a lot more than what is being shared & reported by media. Journalist are legally restricted in respect to publishing things they might find through independent investigation (this is to protect ‘sub-justice’ court/jury integrity)
…I only mention this because this case seems so easily traceable. The owner of the car registration is known with presumably few individual who has access. Clear alibis can be established based on travel movements and tour dates, and no doubt there is loads of CCTV footage in that area
They might arrest someone and file charges with just a provisional death certificate - as long as it says "homicide."
If, however, no direct evidence of homicide is found, the waiting time for toxicology reports can be 6-8 months. Those reports could actually give evidence of criminality (she could have been injected with drugs - and there could be forensic evidence that someone else injected her or witness testimony about who supplied the drugs).
The witnesses may talk if they are confronted with the forensic evidence from toxicology.
I personally believe the autopsy will yield direct evidence of homicide, as I'm sure most of us believe, but we have to wait to have that verified.
And, as to "easiness" of figuring out what happened, It's more than just the "traceability" of who last touched that car (if people were sharing a key fob, which apparently Teslas do have, then all of that has to be looked at and people's alibis verified). It's unknown exactly when her body went into the car, there's what, a two week window? Could be quite a few people to check out, especially given the lifestyle.
It may not be easy to quickly discover who, of the many who might have had access to the car, put the body in the car. But as subpoenas are filled (Tesla will surely disclose what it "knows" from the car's computers and I'm guessing that some of that information is already in the hands of the task force).
Surely this is a homicide task force, even if no MoD or CoD is official, the detectives are surely ones well-versed in homicide. Let's hope the neighbors had cameras, that they were aimed at the street in such a manner as to preserve who parked/moved/interacted with the Tesla from at least as early as the last 10 days of August, when neighbors called in complaints about the car being parked in one place for too long. The lot lines and houses are further apart than in many neighborhoods, I just hope there were no blind spots in camera coverage. They may already know exactly who did it, but they gotta have at least a provisional death certificate to file charges as murder (they could file on lesser charges just to grab the perp, but that can confuse future jurors/the public and isn't considered a great practice).
All JMO.