I think the main difference is the same as between the verbs 'say' (λέω) and 'speak' (μιλάω), one is used to express the fact of articulating a message and the other to implicate the speaking capacity. Correct me if im wrong!
Yes, you are correct. Τι μιλάει το αγόρι means "what does the boy speak" and expects an answer like Ελληνικά, Αγγλικά, Ισπανικά etc. It is a correct sentence but not correct as a translation for this exercise.