The very possible possibility that there has been a long string of missing children's cases all due to the fact that at some point, some poor innocent kids entered the haunted mansion and weren't competent enough to escape the hungry childd-eating grasp of Fangula, Mums, and Frankenbeans.
And if the Protagonists hit the wrong combination of notes on the pipe organ they'd be three more "missing" children.
We never find out exactly what Les Fortunes was imprisoned for, but it was something serious enough for him to be put in solitary confinement. It does seem pretty likely that he had murdered someone.
In one episode, the Hillhurst monsters are plagued by Wolfie's fleas. All right, he's a wolf/human hybrid, the Beetleborgs are human children, Frankie and Fangula both have some sort of undead human blood, but why do they bite Flabber? ...well, he's some sort of seventh-dimension entity...WHY DO THEY BITE MUMS? A mummy literally only exists because it's not exposed to liquid. Mummies don't produce body heat, so why would fleas even TRY to bite them?