there are no ways to infer a feminine pronoun in this case. 'his' must be accepted
why is it "legs" when the sentence is in the singular? (i.e. "hai" and not "haiN")
That is because "Legs" as in a pair. It is considered a singular unit. You don't say pant do you?
no, उसके means his or her, not mine. I think that there is a slight difference between how you would say that there is pain in someone's legs and that their legs hurt. But according to me "her legs hurt" should be an acceptable answer, though i am not sure wether that is only my opinion or generally true...