<<...you gotta TIGHTEN the nut...>>

Right; and DON'T run the engine lean. That Laser 150 only uses a single, non-locking nut to secure a 16x8 prop; hasn't thrown it yet.

Running lean causes backfires, and on a four stroke that will throw a prop faster than a cat can lick its butt.

Gary, I know what you're talking about re. distorting the back plate. All I can say is, you're overtightening. Or perhaps the back of the blades are forcing the alignment pins down (back toward the thrust washer), instead of resting against them.