Hello Dr. Clocknik,
Thank you for the details of the issue you are experiencing with video playback in iTunes. You have taken some really great troubleshooting steps already, but I found an article with additional steps you can take. I recommend reviewing the following article (you can skip any of the steps you have already tried):
Thank you for using Apple Support Communities.
As of right now I have reinstalled iTunes once again, cleared out the AppData folder of any errant Apple files, and attempted to play one of my TV shows.
Same exact result: black screen, three seconds of audio.
It should be noted that when attempting to watch from the cloud, the issue is slightly different: three seconds of audio, then the video plays with no audio.
Hi Dr. Clocknik,
Thank you for the additional information. Due to the nature of your issue or question you may find more information by using another one of Apple's support resources - https://getsupport.apple.com/GetproductgroupList.action
All the best,