Re: Regarding Horses...
So it would seem.
The game was intended for both single and multi-player action. While the skill may be useful in the multi-player settings, there is no benefit to having the skill in the single-player game, nor am I aware of any way to alter the AI to cause NPCs to use horses.
That isn't to say there isn't any benefit to learning the skill... just that there's no benefit to having it. If you train the skill early in the game for 100 gold, one of the Soul Patchers can put that point back into your point pool, allowing you to effectively buy skill points. At one time, it was a well-known exploit to purchase every available skill in the game, regardless of whether or not you intended to use it (or even if, like the unhorse foe skill, it had no use), soul-patch yourself back to 0 in every skill, and then only re-allocate the points back to the skills you plan to actually use. Obviously, that exploit works better when you buy the skills sooner, as your character level determines how much a trainer charges for training you.
As to using the SDK to get the NPCs to ride horses, the AI settings to control various NPC actions are configured by using unique markers placed at specific locations on the world maps. Each task an NPC might perform has an associated marker that you place at the point where the NPC is supposed to perform the task (like placing a marker for forging on an anvil). If the NPC AI is looking for something to do, they'll move to an appropriate marker and start performing that task.
As there is no "ride horse" marker, there is no way I can think of to get an NPC to ride one.
"Streamlining is awesome... if you're developing something which needs to be aerodynamic - like a car or a plane. Otherwise, it sucks." - me