Siapa kamu dan kamu siapa sama saja. But usually indonesia people used 'Kamu siapa?'.
Siapa = who, whose, whom. Can it be translated as "whose are you?" - I believe in some contexts it would be useful, as you are more interested in a person associated with an unknown speaker more than in a speaker itself.
You skipped a valid shortening again. "Who're". A shortening using an apostrophe is STILL VALID.
Probably valid. I've actually never seen this spelled out or used in any text. I almost though it said something else. :)