Relay coil suppression
Q1) For this setup, what power/current ratings should I select for my
  zener and diode? It can be safely assumed that max switching frequency
  won't exceed 1 Hz and it won't be switched more than 10 times in an
  hour. I want to design the PCB in minimum possible area.


The coil will source as much current as it has stored. So if it has 33mA then that is how much the diodes need to be able to handle. Check the power rating on the diode.


  Q2) Is it OK if I replace D5, D6 with 24 V zener diodes? Motive behind
  this is to use a zener array to save PCB space.


The zeners won't start clamping until they get to 24V, in addition to the 12V that they are tied to with a total of 36V. The transistor collector (or VCEO) should not exceed 40V so your still good. 


  Q3) Is it OK if I replace zener + diode combination with a
  bi-directional TVS diode? Motive is to use a TVS array to save PCB
  space. If yes, what should be the rating for the TVS diode? Will such
  a device work?


A TVS behaves like two zeners in series back to back so its essentially the same thing.