Three questions

I have three questions about this film.

1) What exactly does Count Krolock intend to do with the kidnapped Sarah? At the vampire ball he shows her to the other vampires, but to what end? Is she to be killed? Does Krolock want to wait until she turns into a vampire on her own so she can live in his castle?

2) If the villagers are so superstitious and have taken such precautions against vampires, how can Krolock just kidnap the adult Sarah? Didn’t anyone tell her there were vampires nearby and to watch out? Why is there no garlic hanging in the bathroom?
To me, this is clearly a gap in logic, especially in light of a scene from Bram Stoker’s novel Dracula. There, too, the many villagers are extremely superstitious. They are afraid of vampires and of Dracula, and they protect themselves in various ways. Shortly after Jonathan Harker is attacked by the three vampire women, the Count brings them a baby he has stolen. This scene shows why Dracula wants to travel to London – he has a hard time finding prey because of the superstitious and protective population!
In The Fearless Vampire Killers, people are also superstitious – but Sarah seems to know nothing about vampires and bathes in a room without garlic or crucifixes.

3) I also don’t quite understand the ending of the movie. The narrator says that Alfred and the Professor unintentionally brought evil upon the world. I have a few questions about that as well:

Does this mean that there were vampires only in Krolock’s castle and nowhere else in the world?

If a vampire bite is all it takes to turn into a vampire, and the vampires shown, who apparently came from other places, kept biting people – wouldn’t vampires spread all over the world on their own anyway?

Is it in the vampires’ best interest to spread throughout the world? In other words, are Alfred and the Professor really helping the vampires? Because if the world is completely or mostly populated by vampires, that means a lack of food and therefore the end of the vampires.