***** NOT A TEACHER *****
(1) I believe that one uses "did" only for emphasis:
Tom: What stirred up the people?
Martha: There were no free elections.
Tom: I'm sorry, but I disagree.
Martha (a little angry): You don't? Well, then, since you
seem to know everything, what did stir them up?
Tom: Poor economic conditions.
Martha: Yeah, I guess you're right.