does unit heal from zhonya

Yes — Zhonya's Hourglass explicitly heals the saved unit; the card text reads "Heal that unit, exhaust it, and recall it," so all damage marked on it is cleared (Rule 418.1: clearing damage from a unit is Healing).

Key distinction: healing only removes damage. The recall doesn't touch anything else, so buffs, buff/debuff layer alterations, and the Temporary keyword all stay on the unit (Rule 453.1). A unit saved by Zhonya's comes back to base exhausted with zero damage but still carrying its Might modifiers.

One caveat: if more damage is still pending on the chain (e.g. Icathian Rain's remaining triggers), that new damage is marked at base and can kill it again.