Hiding/taking the body after hit and run IMO makes sense if the car / driver was already engaged in something illegal, driving unlicensed, drunk or reckless driving, speeding, illegal drag racing, a drug run? Or if the offender was out on parole or already on the police radar for something else, and desparately needs to stay out of trouble.
It doesn’t make much sense if it was a stolen car as police would not quickly be able to trace the car involved in the incident to the driver - the thief would likely just do a runner and leave it all behind, car and victim, in the bush. Why do thieves like to burn stolen cars though? Just for fun, or to hide their DNA and avoid conviction for car theft?
The hospital theory is plausible, a change of heart enroute…
A more morbid version might be if the victim ended up on top of the hood or roof of the car, clinging on for dear life for a while, or came through the window, or fell into a Ute (pickup) tray etc.
One other thing I thought of - SMs friend interviewed on Ch 7 (linked earlier) said words to the effect “SM is sensible, she would have run in the shade”. If so perhaps harder for a fast joyriding/racing car to see SM, coming around a corner at speed, perhaps no time to avoid a collision and no time for SM to take evasive action. Perhaps two cars racing side by side, making even harder for all to take evasive action. Maybe leading to one hitting a tree? IMO