I think it may be a case of waiting for toxicology.
If she had been dead longer than July, even all the way back to around February, then the condition of her body could be a serious barrier in determining a cause of death.
If we assume the police have much the same footage as the PI. It may suggest that it wasn't D4vd who moved the car. If he had, then they rather straight forwardly have him connected to offences around the treatment of her body which could be a jumping off point to apply pressure.
If it was N that moved the car, then perhaps they don't have direct evidence D4vd commited a crime. The fact it was D4vd's dead underage girlfriend in D4vds car moved by D4vd's lackey to outside D4vds house isn't in itself evidence he was involved, it depends on what they can prove, and he has money for the finest legal advice.
They may want to build out the case to tie him to it, or have what they need to pile the pressure on to N and flip him.
I mean that the PI alluded to the person who moved the Tesla 'leaving town right after' and going straight on tour. So if N went with D4vd to the first show he as well as D4vd would fit the information the PI shared about the driver.
I wonder if N was supposed to come back and deal with her body in August while D4vd was on tour. He is in party pictures back in L.A in that month so he didn't follow D4vd to every tour stop.
But agreeing to and actually dealing with a badly decomposed dead girl are two different things. Maybe he started to get things together to get rid of the body (the disturbing supplies the PI mentioned in the house, and the report of someone inside while D4vd was on tour) but when it came to actually going through with it he just... didn't, or couldn't. So instead, he just longed it off, did a sh*t ton of drugs, partied hard (perhaps on money D4vd gave him?) until their luck ran out and the car got towed.

MOO