Thanks to everyone for all your posts - I just read the whole thread because the story made the national feed today.
Severed vertebrae and ruptured aorta when Meighan's BACK hit the guardrail indicates that she did not jump (a leap would have been face forward) but rather was flung out the door backwards at a fairly high rate of speed. She flew through the air and the whole impact was to her spine, after which she fell over the rail resulting in other injuries in the high speed sequence such as head trauma.
JMO I think MC wanted to get herself and her daughter out of the car. JW agreed and stopped. MC unhooked her seatbelt, opened the door, and at some point JW floored it. As the car entered the curve, MC was clinging to anything to stay in the car, but centrifugal force on the curve threw the door wide open and her unsecured own weight was flinging her out.
I think MC clutched her mother's hair to try to stop JW the wild drunk driver, and that was about the last thing MC grabbed at before she was thrown out.