They do this to keep it fresh. I think it's good. Not good if you're trying to earn the full 5 point bonus, but who cares about that? I'd rather learn something useful at the expense of getting a perfect score.
Sie with upper case S is the polite, formal You to be used when addressing people we don't know or to show respect. It takes the verb in the third person plural, Was benötigen/brauchen Sie? What do You need?
Yes, it is a little disturbing when Duo throw a totally new word at us like 'benötigen'. This is not even one of the words they intro if you actually use a computer and not the app.... but it makes me remember it because of the shock value. ;-)