If I were going to do anything to improve the prop shaft, it would be to cryo-treat. There is no dimensional change and it re-arranges on a molecular level. In todays world you don't have any idea where the raw material comes from or where it is centerless ground, so the quality is always suspect. If you can find this material made in the U.S. (or maybe Germany or England) and ground here I would pay the premium. Some of the photos of keyways appeared to be the coupler end, have them done by someone you trust your life to. Machining these materials sucks and not every machinist has the skill, tooling, or patience to do it correctly.