1. In C2 you can have event sheets to control things globally - if that's what I think you ask for.
2. What alextro said. You can have event sheets to set functionality for specific objects and simply include them where you need them.
3. No. There is no visual representation of all layouts in the project. All of them are visible as a list in Projects panel window.
4. There is no limit for layouts or layouts size.
5. OBB, if you mean extension files (for games larger than 50MB) then XDK apparently is working on that.
6. 500mb yes. Huge games... it depends what do you mean by huge game. Big layouts? Lots of textures?