Quote: "2. Can you make multiplayer teams?"
Yes, with mods you can.
Quote: "4. Can the player drive vehicles?"
I believe there is one driable vehicle in TGC store. It's a motorcycle to be exact and it's really cool.
Quote: "5. How much can you edit the start screen, game over screen, etc..?"
As much as your skills allow you to. And trust me, it's really easy to do this.
Quote: "6. How much can you edit the water in the x9 version?"
Height only, but in the v1.18 beta thread, rolfy posted a new normal map for it, so you can check it out too, if you think it looks cooler. But remember that the water is still in a beta stage.