1) I wouldn't use it in writing such as in essays, because it's normally used when you're speaking to someone in particular. You can use it while, for example, writing a letter, an email or a text to a friend. Otherwise, if you have to write a formal letter, you'd want to use "Mi dica", instead. 2) "Dimmi" and "Diceva tu" don't mean the same thing, and, actually, "Diceva tu" is not even something you would here in Italy, first cause the verbal conjugation is wrong, second because we put the pronoun first and then the verb, so it would be "Tu dicevi"; said so, "dimmi" is present tense of the imperative form ("tell me"), while "tu dicevi" is past tense of the indicative form, and i think the closest way to translate it is either "you were saying" or "you used to say". So, yeah, there are some differences. 3) No, this form doesn't exist in italian.