#4 was an idea that Cameron Crowe had that apparently Amenabar didn't think of until Crowe mentioned it to him. Amenabar happened to like the idea a lot. This is revealed at the end of the commentary track on the Vanilla Sky DVD.
By some stroke of luck, I was able to find this DVD (of one of my top 5 films of 2001) earlier this week here in Sweden.
I haven't had a chance to watch it yet, but I'll do so tonight. Glad to hear how great the disc is, Ron.