1. They came in 3 other colors in multiplayer games.
2. Who would he talk to? Himself?
3. They weren't "inspired" so much as taken right from a picture of some "sugar loaf" mountains in China.
5. Dunno. Can never get a firm answer about this.
Check out the photo archive at Rome.ro for some shots taken during Doom development. His way of celebrating the 11th it seems. Looks like we'll even see some from Commander Keen (!) development next week.