ok Sirius Black isn't dead nobody knows but J.K. he was hit with a red spell blocked it then hit again but we dont know by what he fell through the portrait. I'm saying he got stunned and fell in the portrait. Later in book 5 Harry got told that the reason that portrait was in the Department of Mysteries is because what ever fell through there never returned. well in book 7 i'm guessing Sirius comes back because he finds a way out or somebody at the Department does. You say "why would J.K. bring him back anyways" why wouldn't she he's much more help to harry then two 17 year old teenagers