What is the explanation of the hadith, "Baffling is the kinsman of the tribe," and are there any hadiths that permit backbiting?

"Woe to the brother of the clan" is a description the Prophet, peace and blessings be upon him, used for Uyaynah ibn Hisn, to show people his true state. He had not embraced Islam with sincere faith, despite outwardly professing it, and the truth of this description became evident with his apostasy. The Prophet, peace and blessings be upon him, spoke gently to him to win him over. Such a description, which includes permissible backbiting, is supported by evidence in the hadiths, such as his saying, peace and blessings be upon him, about two men: "I do not think that so-and-so and so-and-so know anything about our religion," and his saying about Mu'awiyah and Abu al-Jahm: "As for Mu'awiyah, he is a destitute man with no wealth, and as for Abu al-Jahm, he never removes his stick from his shoulder." This is also supported by the Prophet's, peace and blessings be upon him, approval for Hind to take what was sufficient for her and her children from the property of the stingy Abu Sufyan.