Why don't you donwload it? You will have all your answers!
1) and 2): no multiplayer
3) (map editor) You can only decide of the places where the players start, the relief, the ressources, the trees, the stones and the texture of the ground.
To add new models to the game, you have to make them with 3D studio max, then you must to convert it into .g3d format with the plugin that is in the pack.