Wanted to mess up her own career, because it was almost over already, as it is, that she was almost 21 years old. And, according to Marinna, modeling was a cruel buisness because besides it not wanting you once you weighed more than 118 pounds, they also didn't want you once you were 22 years old.
The second mystery nods to classic seventies exploitation horror without of course the lascivious shower scenes and such. Scooby-Doo is after all an all ages book. The setting however represents a tradition of the genre.
The "beast" is again an imaginative creation, and while a summary of exactly how the culprit carried out the crime would have been welcome. After some thought, you can see that it's not absolutely necessary.