Aetherling's first ability will return it to the battlefield only if that ability also exiled it. If Aetherling left the battlefield in response to that ability, it won't return, even if it was exiled by another spell or ability.

Aetherling's last ability can be activated even if its power is 0 or less. If a creature would assign 0 or less damage in combat, it doesn't assign combat damage at all.