The decomp can the jury question is risky, for all the above mentioned reasons. It further has potential to backfire though in that it could make jurors wonder why this car stench was being ignored by people and how nutty would you have to be to withstand a serious stench of your child decomposing as you drove around.
It could have effect of lending some support to her living in a deep state of denial and out of touch with reality due to habitual abuse etc.
A body in a car can't be missed too easily. A man I dated lost his father to suicide, committed in a minivan with a shotgun. It was a rather traumatic wound to the head that left the vehicle a mess, to say the least. Nobody would clean it up, not the bank that owned the note, the widow couldn't bring herself to do it and the insurance company said it wasn't their prob either. The neighbors were ready to storm city hall within two days the odor was so pervasive, even under a tarp parked at the curb.