About that splitting up thing, I have thougt about it for a while, and I think that I have figured out a way of doing collisions to both walls and floors without the player being able of walking on too steep walls.
In addition to the normal player collision sphere you will need another sphere, one with a small radius. It should be located right on the bottom of the other collision sphere. The lower sphere will only collide with the enviroment if the surface is not too steep. So by checking the small sphere for collisions you can tell if the player can walk and stand, or if he should slide down the hill. I havn't written any code using this, it's just an idea. I hope you can use it :)
In addition to the normal player collision sphere you will need another sphere, one with a small radius. It should be located right on the bottom of the other collision sphere. The lower sphere will only collide with the enviroment if the surface is not too steep. So by checking the small sphere for collisions you can tell if the player can walk and stand, or if he should slide down the hill. I havn't written any code using this, it's just an idea. I hope you can use it :)