A: The module system does not provide enough control over the rendering process to implement split screen in a mod (proof me wrong). Top down perspective was the only playable alternative for me.

A: Because it is technically something completely different. Try the mod "Battletime", it is the best you can get for online co-op. A real co-op mode is an old dream of the Mount&Blade community, so why isn't there a mod that satifies this hunger? Because it is a hell of (unpaid) work for modders and the limitations of the module system make it almost impossible. You would have to write your own netcode on top of the game and host your own server. Trust me, you are not the first person with this idea, go and search the internet to learn why it is so hard to do.

A: I plan to implement riding for player 2 someday, but apart from that, I don't think so. If you really want some features, look up the "Contributing" discussion, the sources are online. You could write it yourself, if you want to.

A: Gamepads should work, but it doesn't seem to be possible to override the main player controls. Also, you can't use the gamepad analog sticks, as it isn't supported by the module system. You could try something like JoyToKey.