Another way a bullet could've ended up under Jackie is that if the head shot came from the front. After being hit for the first time JFL sort of lolled over to Jackie, his head almost on her shoulder. If the head shot came from the front, a bullet could've ejected down the back of her seat.
Just seems to be very odd how a bullet would, if we accept the lone shooter/TSBD narrative, end up on Jackie's seat. And did Mr. Landis ever mention this at the time and if he did why wasn't he called as a witness to the WC.