<<why Vishnu is considered the preserver, Shiva is considered the destroyer, and Brahma is considered the creator.>>

the vedic literature says so.


<<Isn't Vishnu the supreme Godhead? Isn't He responsible for all three of these things?>>

yes. god is one. he does different functons with his different personalities.

<<how the tradition came about for each village in India to worship one specific God only.>>

are you starting a rumour?
I have not heard it.
however, it could be true for some villages.
is is bothersome or fiightening?
if not, why worry about it?


<<do any villages in India worship Hanuman or Brahma? >>

almost every village or city has hanuman temple(s).
brahma is not worshipped
for a reason given in shiva purana.
however, the four heads of brahma are put on the top of a temple structures above the main deity room.

jai sri krishna!