Do Bloodreavers gain an extra attack from a Bloodsecrator's Rage of Khorne ability?
As with most rules in AoS the simplest solution is usually the correct one and rules stack.

Therefore if you have a unit of BloodReavers:


within 12" inches of a Bloodsecrator you get 3 attacks
between 12" and 18" of a Bloodsecrator you get 2 attacks
over 18" from a Bloodsecrator you get 1 attacks


The Chieftain gets an additional attack on top.

I actually wasn't 100% of this so I asked on TGA. Here's the quote directly from Killax:


  Frenzied Devotion gives them an additional attack if they are within
  12" of  Chaos Totem. This ability is not cumulative. The Bloodsecrator
  has the Chaos Totem Keyword but the Bloodsecrator is not the prime
  Keyword for Frenzied Devotion to be activated. 
  - Rage of Khorne is an ability that the Bloodsecrator only has if he 'activates' Portal of Skulls. This is something you have to declare in
  the Hero phase and not something that continues once you do. This
  ability is cumulative because it adds 1 to all melee attacks.
  
  So depending on wether the Bloodsecrator has activated the Portal of
  Skulls or not the Bloodreavers will have two attacks instead of one if
  they are within 12" of the Chaos Totem (which can be a Bloodsecrator)
  and depending if the Bloodsecrator has used the Portal of Skulls or
  not you can add 1 attack in the Combat phase thanks to Rage of Khorne
  if they are within that range aswell. 
  
  The common mistake I see here is:
  
  
  Frenzied Devotion does not look for Bloodsecrator Keyword but Chaos Totem Keyword.
  Portal of Skulls leads to the Bloodsecrator having the Rage of Khorne ability. It is NOT a permanent ability.