Now to rotate the torso.  I’m thinking of mounting the spine into a bearing then I’ll have free rotation back and forth. But I can’t think of any type of mechanical device that is small enough to do this.  Should I use two small motors with a clutch that rotates him then free spins the opposite direction such as having a motor on each side?  Should I use a chain driven motor that will run both ways?  Is there a really simple way that I’m overlooking?

Try an inexpensive "lazy susan" for the rotation platform.  The motor used could then turn "lazy susan" by friction drive similar to the way they turned the old phonograph turntable.

Using the lazy susan idea - attach the torso to a lazy susan.  Attach a length of fishing line on the side of   the lazy susan that faces the audience.  Attach a spring to the other side of the lazy susan.  Attach the spring and the rope to the bottom of the coffin below the lazy susan.  You'll have to play around with   the length of rope that is needed.  It should be adjusted so that when the corpse is sitting all the   way up the side of the lazy susan that has the rope on it is turned toward the bottom of the coffin and the rope is pulled tight.  The end of the spring may need to be moved around in the coffin until you find a spot   that will cause the lazy susan to return to the normal position as the corpse lies back down.

How about a spring and a rope?

Great suggestion. Using a bike cable you can affect that very simply.  One cable comes from left side of ribcage to the spring and another comes from right side to the actuator or ram. Set the air pressure very low for a slower movement. With a very lubricated bearing you can fine-tune the movement to some degree.