Sandra is some sort of demon/succubus.
Jake seems to immediately fixate on her, to the point she’s the only girl he’s really looking for at the party - for a dude so fond of sex that he’s bedded sixty-seven women, he can’t be that picky. Obviously, she has the decidedly inhuman ability to impregnate men, creating monstrous babies her parents seem afraid of, even if they don’t immediately seem scared of her. My theory is that she is some kind of creature that trawls college freshmen events, finding the horniest men around and giving them a Secret Test of Character. The Mortuary is a purgatory.
In the original Roman Catholic sense of a place to atone for one's sins. Darke didn't sufficiently atone for whatever sins trapped him there (perhaps he shouldn't have been hanging out "Help Wanted" signs to find someone to take his place before his sentence was up), and so he was smote with divine wrath when he left the mortuary. If he had fully reformed and served his sentence, perhaps he would have peacefully Ascended to a Higher Plane of Existence. As for Charlotte, who is actually continuing to sin while trapped in the Mortuary, there's little doubt that her Karma Houdini Warranty will expire if and when she ever manages to leave.