Ok so he made the alibi he drove her to school right? He’s got her propped up in the car. Ok. When he’s got back to the backpack dump site, what’s he done with her then? Is she still propped up? Ok. Does he leave her propped up and drive for awhile and find a spot to park with her dead in the seats propped up? Cover her up with a blanket or otherwise obscure her from view to get out of the area? I don’t mean to be seeming like I’m picking at your post. I am not at all! Your post just made me think of a way this may have gone down is all.
Maybe this is how they came to believe she was dead at 8:19. It could be as simple as he did the perp drive to / away from the school with her propped. Can’t drop her. She can’t move. So goes to the home having made the alibi… gets a blanket or whatever and drapes it over her. If he did that - it would have been pretty sus that she was dead.
Driving along with a literal corpse has got to be a weird thing! Maybe he needed some “space” or to depersonalize her a bit in order to make the drive?
Just a theory. Not fact.