Sibella was the most resistant to the brainwashing and had to be forcibly dragged to the castle before it could set in. In some stories, vampires have hypnotic powers. In addition there may have been two other factors.
She is the oldest of the girls, meaning she has the most developed mind that is probably not as susceptible to mind control as the younger girls.
Sibella's implied to be a vampire/human-hybrid—it's possible that Revolta's powers just aren't as effective on humans (and/or those with human blood) than they are on full-blooded monsters.
Tanis having blue eyes isn't so strange when you consider how much ethnic diversity Egypt has, as well as how Egypt had European slaves and contact with Europeans for centuries.
In the scene when the fathers show up, it would make sense that Tanis' wrapping is a very strong white color while her Mummy Daddy's is an off white, nearly yellow color. Since her father is so much older, his bandage wrapping should be more faded and yellower.
It might seem odd that, as a gym teacher, Shaggy would have had to obtain a college degree to gain qualifications. However you can look at it as Velma, who both Shaggy and Scooby are close to, rubbing off of him. It is also possible that Grimwood didn't ask for a degree, though that is made questionable by the fact that Shaggy does seem to know what he is doing.
It's unclear if it was intentional or not, but making Elsa be a genius despite her simpleton voice could be a way to combine the way Frankenstein's Monster is portrayed in the movies with the book version of him.
Shaggy and Scooby being certified gym teachers here can make sense if someone finally pointed out their Accidental Athlete prowess to them.
While the original Grimwood Girls are all references to classic gothic horror (Frankenstein, Dracula, The wolf Man, The Mummy, and ghosts), the new arrivals at the end of the movie reference the newer era of "Atomic Horror" (aliens, mutants ala the Gill-Man, and kaiju like Godzilla.)
Mrs. Grimwood and Colonel Calloway both seem to run Two Teacher Schools (although Calloway may have more staff members offscreen) with only too few students to have normal-sized volleyball teams. One possible explanation for this is that the Grimwood girls scare away a lot of boys who would otherwise attend the military school. Furthermore, given how seriously the Grimwood families seem to take the volleyball tournament and how long it has been since their school won it, it is possible that other, even more athletically-minded monsters pulled their girls out of school rather than see them keep losing the volleyball tournament. If one or both of these possibilities is correct, the lingering bitterness could help explain the heated rivalry between the schools.
Neither Revolta nor the Grim Creeper are seen again after they're shot down by lightning while on their brooms. Even if they survived the electric shock, they were still floating above their lair as it was exploding.
For that matter the mirror monster, the Spider Bats, and the (fairly sympathetic) well-dweller weren't shown escaping the castle at all when it exploded (although perhaps the Well-Dweller was safe underground). The mirror monster may live inside another dimension, which might have given it some protection even if the mirror is destroyed, while the spider bats might have been able to hide in the books and crannies of the castle if they are more spider than bat, but it is also possible they did indeed die.
