Well, if that's the case then the FBI and the White House are colluding with Sony to cover it up, because even if Sony lied to the FBI, by the time they were done combing over Sony's network they would know about it. Also, even if money was demanded, it sure must have been an astronomical amount, because I'm sure Sony would have happily paid anything semi-reasonable to avoid this disaster going public.
I'm not saying what you're stating is impossible, but it paints a conspiracy that involves the government that would make themselves look bad, which doesn't make much sense. From a governmental PR perspective, its a lot less damaging to make the claim that this was done by some hacker group than it does to lay the blame on North Korea.