I have the german version. On the elder vampire encounter card it says something like: "the hunter may become delayed to discard this card".
Who is deciding this ? Dracula or the hunter ?
It is the hunter's choice
What is the thematic explanation for clearing hideouts 4-6 on maturing vampires ?
I don't know of a thematic reason, the gameplay reason is to prevent Dracula lining up a row of vampires that can be matured in quick succession.
How are revealed encounter cards handled which stay on the hideout like rats. When do they trigger again ? Why should hunters desire to move on this location again to trigger them ?
They are triggered in the same way that they were previously triggered, that is by Dracula ambushing an arriving hunter or a hunter searching. In most cases the hunters would not wish to trigger them again, but maybe they need to move through the city on the way somewhere else or they may want to prevent the card from maturing (not the case with Rats, but certainly the case with a vampire).