Sweet spot in relation to the shaft for Drag Loaders vs. Driver Loaders
 

Why does the Drag Loading work better with having the shaft and the longitudinal center of gravity in the same plane? Does this more effectively load the shaft lengthwise rather than stress it sideways?

The design of the club. To most efficiently us it to transfer energy, you have to allow it to rotate - or - counter that rotation. In either case, the root is to provide the most 'on plane' loading for the physics in use, which in turn provides the most 'on plane' support for impact (lag pressure). The relationship between speed and mass in generating force - swing vs hit respectively.