According to what I understand num is used when the asker of the question is already aware that the answer will be no. Does this have any other uses besides rhetorical questions?
Num can also be used in questions that expect the answer "no", even if they're not rhetorical.
Num pater eius es?
Surely you're not his father?
You're not his father, are you?
You could certainly reply "yes, actually, I am" to this question (immo in Latin); it's just not the answer the questioner expects.
For direct questions, num is one of the the explicit ways, together with nonne and -ne, of signalling a yes/no question. Whereas -ne is neutral and simply asks for either confirmation or denial of some detail (that is, the respondent can reply either 'Yes' or 'No'), nonne and num are used to ask leading questions.
When nonne is used, the person who's asking the question expects to receive the answer 'Yes' or 'Yes, of course'; and, as you've noted, when num is used, the answer 'No' or 'No, of course not' is expected. Questions with nonne are sometimes translated by saying something like 'You did see Cicero, didn't you?' (nonne Ciceronem vidisti?), questions with num by saying something like 'Surely, you didn't see Cicero, did you?' (num Ciceronem vidisti?).
For indirect questions, num loses its negative leading force. Both it and -ne just mean 'whether' ('if') (nonne retains its positive leading force though).
rogavi num Ciceronem vidisset = rogavi Ciceronemne vidisset.
I asked whether he had seen Cicero.
Although Allen & Greenough, New Latin grammar says (§332, b, Note) that num in indirect question 'commonly loses its peculiar force' (emphasis added), I don't remember ever seeing instances where such a num had to be construed with negative leading force.