And you could say the same is true if you lost an AES key. But if they can establish a chain of evidence that shows (to whatever degree the court you're in requires) that it does contain the work, you've lost.

How many ways could they do this? Could they note in court that they found you getting your copy from a "super secure no liability legal loophole" piracy service? Could they just get B's side, whether through subpoena or whatever mechanism you have to communicate with B? (You must, since your file is "just noise" and useless to you as it is)