Symmons sent the replacement seats and tools to repair a leak from the diverter hole that happens only when water is turned on. I replaced the seats, tightening by hand stopping short of cranking them. (I didn't use a torque wrench).

Three times I removed them and adjusted them, but the water continues to leak from the diverter handle hole when the water is turned on.

Replaced the spindle. No difference. Put the original spindle back. Pondering whether I have to make a call to a plumber?

Outside of the exact 15 foot pounds of torque, what could I be doing wrong?

Well since you already gave me a reddie on someone else's post, which I was correct by the way, I guess you should call that plumber because I'm fresh out of ideas on how to help.