1) Harry rules out his father or his friends as possibilities for the Half Blood Prince based on the printing date of the book. Now this is slightly shaky logic because it assumes the prince was the first owner of the book but if we accept it then doesn't that also rule out Snape? He was at school at the same time as they were. If we rule out Snape on the basis of timing, I suspect Voldemort to be honest. Could even be another horcrux.
2) Kreacher and Dobby apparate and disapparate inside Hogwarts. I don't know why I've never noticed this before. Does the prohibition not apply to house-elves? Can either side use that information?