Full Manual Turntable-What to do

If the main argument is saving the motor /switch vs. saving your vinyl?

Neither, the argument for leaving the motor on is that the vast majority of motors take a long time to fully speed stabilize and hence you are best of to avoid turning them off if you can avoid it. Provided your mat is not "sticky" then there’s no real risk to the disc in taking it off a moving surface. In the case of my deck where the platter is coupled via magnets to the motor I can stop the platter while leaving the motor running so I tend to leave it on

Obviously removing a clamp with a spinning disc is a recipe for trouble. And if your deck is designed for a clamp then sure use one. My experience is however that for decks not designed for clamps they can deaden the sound making it slow and heavy. The one thing I do do is to use thin washers to ensure the center of the label is supported as my deck has a label cut out and depending on the disc and label thickness the center may float. This makes a subtle difference