Pandit,
Good post. I don't know, but I can't imagine the back seat working it's way out of a window. Perhaps it could float out of a door because the door's wider. If the seat was lightweight, it would certainly float further away from the car. Some of the older cars took a bit of force to pull the back seat out.
Hi Canning Vale, It was my understanding that only the drivers door was open which would mean there were front seat obstacles to get past so working it's way out lengthways first through the broken rear window, which is quite big and squarish on the older Fiat's was how I envisioned it's exit. Julie's Fiat likely had the standard bench seat in the rear, separate from the back rests.
Not to suggest though that Julie wasn't a victim of foul play because imo I think it likely that she was.