Destruction only.This unit gains the power of the topmost unit in your discard pile.

If the topmost unit has an ability that gives it more power does this count.

Or is it just the printed power on the card that Dark Acolyte gains?

Usually it will be printed power, constant effects boosting unit's power doesn't work in discard pile.

A card text that determinates the value of "X", is that a constant effect ? No "Action", no "Forced"... seems to be constant.

If so, how can or should you play Shaggoth Champion (from hand or discard pile, with necro-countess), since "constant effects only apply to cards in play" ?

X isn't a constant, This unit gains [P] equal to X is a constant. X is defined even in discard pile.

Though the x was defined by a constant effect : "X is the fewest number of cards in any opponent's hand. "

So it's just card text, and not an effect.

Thank you