I don't think that a sentence with fronting modal verb can express an imperative.
The optative (i) expresses a vague wish without an agent: ‘Oh that I may have a mango’ and there is no inversion, and (ii) no answer is required or possible to the optative.
The interrogative has inversion and implies a distinct choice of answer; it ends with a question mark, even if spoken sternly so as to imply "Get me a mango!" If the exclamation mark is to be believed, then it is not interrogative.
The unmentioned "rhetorical" implies a particular answer and, depending on your view, perhaps requires no question mark:
Rabble-rouser at the hustings: "The king is starving us! May we have a pork chop?/! May we have a loaf of bread?/! May we have a mango?/! Are we to die without a fight?/!"
The answer to the question then becomes one of punctuation, context, and tone... the former is unexplained and neither of the latter is mentioned.