When is Lashon Ha-ra permissible (or obligatory)?

At the close of our Sedra, Miriam is punished for speaking negatively against Moshe, who had separated from Zipora, due to Moshe’s need to be available & in a state of ritual purity at all times. (Note that Miriam spoke only to their brother Ahron). Only in certain situations may one speak (ill) about others: 1) In discussing X with someone (e.g. therapist, dr) who can help X’s problem; 2) To prevent Y from being harmed by X (e.g. a sexual molester or con man); 3) To help end a dispute that harms the wider community; 4) To help others learn from Y’s mistakes (e.g. "Don’t invest with X like Y did; Y lost all his money"). Even then, one should utilize other means if those are available; the speaker should be acting out of genuine concern for the subject or others & not to exact revenge or to shame the subject; the information should be true & accurate. If there is the possibility to speak to the subject directly, rather than about him, then that is the preferable option.