So here we go. The only unit cards in play are my opponent's Night Runner and Clan Rats, both which feature ACTION: CORRUPT THIS UNIT TO... abilities. If I attempt to play Seductive Delusion, regardless of which unit I choose to corrupt to damage/slay the other, can my opponent take the target's action in response, corrupting it, to cause Seductive Delusion to fail (as it cannot corrupt a corrupt unit)?

Yes. When an effect uses the word "then", as Seductive Delusion does, the part before "then" must have resolved before the part after it can resolve. This is not possible here since you indeed can't corrupt an already corrupted unit.