Will I be absolved of responsibility if I repel backbiting someone by saying, "I believe this person is righteous," and must I then clarify the ruling on backbiting to the one who was backbiting?

What you did by defending your Muslim brother is legally required. It is stated in "Subul al-Salam" that whoever is present during backbiting must defend his brother's honor, or leave the place where the backbiting is occurring, or denounce it in his heart and dislike the statement. As for teaching the backbiter the حرمة (prohibition) of backbiting, if that is before he asks, there is a difference of opinion among scholars. However, if it is after he asks, then clarifying the Sharia ruling becomes obligatory.