Sheila would have been in the same lain down position for both shots. It would have been that she just expected the one shot,but being a semi-auto. the second one would have literally been seconds after. Sheila wouldn't have had to have moved an inch. Because that first shot hit the jaw-bone it would have naturally followed that a second shot an inch or so away from a non-moving target would occur.
For that second shot to have reached the skull,she'd have been lying flat and the rifle pointing upwards under her chin/neck.
The first shot was at a 45 degree angle to her neck but the second shot was 90 degrees, the first shot was front to back of her neck and slightly upward, the second shot was from a different position ie flush along her body or lower than her body with her lying flat with her head on the floor so the bullet travelled up through the underneath of her chin through her mouth and into her brain.
My thoughts are that it would be unlikely a reflex second shot would do that? I may be wrong but that's the way I see it.
We know if a third party was responsible for the second shot they would have needed to hold the rifle against her body with the end touching the floor below the body, so how else could this happen than by being placed?