Hello

If I take unit from op. discard pile, put in in my qz can I return it to my hand from har ganeth/alrat of Khain ? (when has 1 hp / is destroyed).

The "owner" of a card never changes. If a card is in your deck at the start of the game, you "own" it. Har Ganeth specifically states that the unit returns to its owner's hand. Altar of Khaine requires you to target one of "your" units, which I would say denotes ownership, not control.

RM

All instances of the word "your" refers to control of a card, as in "....all corrupted units (yours and your opponent's)....". However, when a card leaves play, it returns to the specified out-of-play zone of the card's owner, be it hand, deck, or discard pile, regardless of control. For example, if you take control of a unit and then bounce it "to hand", it returns to the hand of the card's owner.