Increasing mainsheet tension decreases rotation and twist, but not much.

With regard to the spinnaker affecting the mast rotation, it should not have much affect with the proper set up. Keep in mind that the top of the spinnaker should be rigged with a metal bail or rope system that allows it to rotate freely around the front of the mast.

See this URL for further pictures, diagrams and explanations:

http://www.geocities.com/f16hpclass/Tech_genaker_mast_setups.html