1: Having a ship with zero max velocity and ample turning times and something like a fighter or cruiser ship flag should work. You'd give the sentry gun an attack order and it would turn to face the target and fire when it can.
2: Well you can't directly copy the movement, but you can try and fake it... What I would do there is have an asteroid-with-turrets ship and give it the ability to glide. Then set the initial velocity to be something that would match the rest of your asteroid belt (probably want something pretty slow so it won't exit the field bounds) and force it to glide (with force-glide) and then use the maneuver sexps to give it a slight tumble while it moves.
If you want the asteroid to be on waypoints, and still want a tumbling asteroid, then you'd probably need to set something tricky up in PCS2, like have an invisible base ship, then the asteroid as a subobject that will rotate slowly, and then the turrets on top of that.