Is participating in a gathering of backbiting and reviling, while ignoring the speaker, considered sharing in the sin? And is commenting on a person's actions and mocking them, based on false assumptions, considered an assault on their honor? And how can one expiate these sins?

You should not sit with someone who backbites without condemning it, otherwise, you become a partner in sin; because backbiting is forbidden to mention, listen to, or approve of. Whoever hears backbiting must condemn it with their tongue if they do not fear harm, otherwise, with their heart and by leaving the gathering. As for your speaking about those who openly commit evil, this is not forbidden backbiting. The expiation for sins is repentance to Allah.