Stern does NOT count as a GK Hero, so you may have both Stern & a GK Hero for your two HQ slots. Lord Coteaz is the same - he doesn't count as an Inquisitor Lord for purposes of filling FOC slots.
The Grand Master *is* the Hero. The retinue is just a bodyguard purchased with the Hero. If the Hero can buy one psychic power, that means the Grand Master is the one buying & using it. The bodyguard may buy Holocaust even if the GM has already purchased a power, because they use the rules for an Elite GKT unit. If you meant something other than that, I can't tell, because you have some terrible grammar. Please use a spellcheck in the future.