There was never a director's cut. There has never been a cut other than what you see. In fact, I believe Fincher had final cut on the movie anyway. There was a SPLIT second of a very close-up shot of Gweneth but that's it. And the head isn't in the box. And I don't know how your brother would remember a few extra seconds about "the guy in the bed" if nothing specific happened in those extra seconds. None of your post makes sense.
Also, the Biggs scenes weren't "released" until a few years ago. Before that, no one had seen them besides the people who had worked on the movie so your theory there doesn't work either.