What is the resort of Nibbaana?
In my opinion, "resort" is a bad translation for patisaranam (literally, something other things rest on) -- a better option in this particular context would be "apex". Compare with following simile from Milindapanha:


  Like in a building with a pointed roof: whatever rafters they are, they all rest on the apex, lean towards the apex, meet at the apex, and the apex is pointed to as their chief. So is the samadhi in its relation to the other wholesome dharmas.


It does make more sense this way, doesn't it:


  Brahman Unnabha: As the five faculties are of different scope and range, and do not share in each other's sphere of action, what is their apex, and who profits from their combined activity?
  
  Buddha: Mind is their apex, and it is mind that profits from their combined activity.
  
  B.U.: But, good Gotama, what is mind's apex?
  
  B: Mind's apex, Brahman, is mindfulness.
  
  B.U.: Then, good Gotama, what is the apex of mindfulness?
  
  B: The apex of mindfulness, Brahman, is liberation.
  
  B.U.: Then, good Gotama, what is the apex of liberation?
  
  B: The apex of liberation is Nirvana.
  
  B.U.: Then, good Gotama, what is the apex of Nirvana?
  
  B: That question goes too far, Brahman.