Losing the player board ability in the middle of the turn
The relevant rules:


  It may happen that a Taoist loses his  power because of a ghost
  ability. He can recover it as soon as the  responsible ghost is
  removed from his board.


I can't find anything in the rules that explicitly states when you lose your power, but I would assume that if you get them back instantly, that you can lose them instantly as well. 

Regarding the symbol on the ghost card:


  As long as  this ghost is not defeated,  the power of the board  where
  the ghost card has been placed cannot be used (place an  Inactive
  Power token on the power stone of the board)


Even though this symbol might be in the immediate actions part of the ghost card, it is a ongoing effect that transfers to a new board when it is moved.


  The blue Taoist can act in a heartbeat. He may request villager’s help
  AND attempt an exorcism in the  same turn, in any order


This isn't explicit either about when exactly your power is used. If the blue player's power is such that the use of the power occurs before the attempted exorcism, or the use of a village tile, than I would say that you can in fact still do both actions. But this seems to be a strange interpretation to me. Since every player in the game can already can use a village tile on their turn, I would interpret the use of the blue Taoist's power to be when he tries to exorcise a ghost after having already used a village tile.

Therefore, I agree with your interpretation that you cannot attempt an exorcism because you need to use your power in order to attempt an exorcism after having already used a village tile.