Do not understand the Youtube fan clutch test. What was that supposed to show???

You would test your viscous fan clutch with a rolled up newspaper. If it stops the fan blade, your viscous fan clutch is weak or bad.

The old water pump has the proper cast impellers; hoping that the one you installed has the same. Some new ones have impellers which are not cast - they look more like slanted metal fins and are not as durable. I do wonder however about the scoring; do not remember seeing that when I replaced my water pump about ten years ago (as it turned out the original water pump was 35 years old and was still perfect - the squealing that I thought came from a bad water pump was just a bad belt).

Interesting thread.