Making it walk to places (no fancy pathfinding or anything complicated, just simple "look towards objective - move forward"), either to another object on the planet surface or to a mouse click on the surface...

Shouldn't be too hard. If the object is aligned according to its spherical co-ordinates rather than the normal beneath it, you can just translate the destination into a position relative to the object's orientation (vec_rotateback) and rotate the object's pan (relative to its z axis) towards that point. Since you use actual surface normals, we can set up a temporary angle that uses the spherical co-ordinates instead.