CMOS logic Gates XOR
If the only criteria is that a combination of gates be used then there could be an infinite number of 'correct' answers. However, 


  what is the point of using two inverters? Why can't I just draw a
  buffer?



A single buffer is only one gate, not a 'combination'.
In practice a CMOS buffer is made from two inverters, so the answer given is the simplest combination of 'elementary' gates. 



  And why does the second inverter need to be active low?


The negation would be equally valid on the output, but having it on the input shows that the signal is 'active low' at this point.


  For the second function, same thing, why does the output need to have
  an active low inverter? why can it just contain an OR gate at the
  output instead of a NOR and an inverter?


Same thing. A CMOS OR gate is already a combination of a NOR gate and an inverter.