Is it sufficient to repel backbiting by mentioning the good qualities of the person being backbitten and offering excuses for them, instead of explicitly forbidding backbiting?

What you did by mentioning the good qualities of the person who was backbitten is a form of defending your brother against backbiting. The Prophet (peace and blessings of Allah be upon him) encouraged this action, for whoever defends his brother's honor, Allah will protect his face from the Hellfire on the Day of Resurrection. Defending against backbiting is more effective if your mention of the backbitten person's virtues contradicts what was said about them.

However, you must remind those present about the ruling on backbiting and its punishment. If they do not stop, then you are not permitted to sit with them until they cease backbiting. Whoever hears a Muslim being backbitten must defend them and rebuke the one speaking, and if the latter does not respond, then it is obligatory to leave the gathering.