As noted under Artistic License – Religion, Thorn being 1/16th Wiccan is complete nonsense. However she still clearly practices some elements of Wicca which arguably still makes her the best chance to defeat Sarah.
Also, Ben Ravencroft first heard about Mystery Inc from their solving the mystery of the "Moat Monster". The Moat Monster was the same story that Daphne brought up in her interview towards the beginning of Zombie Island, which means The Witch's Ghost takes place in the same universe as Zombie Island.
Confirmed in Alien Invaders. At the end of the "Scooby-Doo, Where Are You?" theme song in that film, when Scooby and Shaggy tumble in to the diner, if you look to the left, you can see the tourist with the hat, brown, plaid shirt and blue jeans from this film watching Scooby and Shaggy's entrance.
The book containing the spell to imprison Sarah despite being her own spell book and clearly evil makes sense when you remember the book can be opened without freeing Sarah, she has to be actively summoned via a spell to escape. It's entirely possible the ones who sealed her were the ones to put the spell in there in order to make sure she could be resealed if she were ever to escape.
Sarah Ravencroft makes one hopeful that she was the only malevolent witch out there. If not, then let's hope they're all similarly sealed away as well. If there are other practitioners of the dark arts, who's to say that somehow, they couldn't figure out a way to somehow bring back the Ravencrofts?
Played for Laughs, but the surely four-figure tab that Scooby and Shaggy racked up is presumably not going to be paid off now that Ben is gone. Let's hope the town gives Jack some of the profits from the festival, otherwise Jack's business is really gonna take a hit from Scooby and Shaggy's feast.
