Further problem is the slot head of the screw that holds the plastic handle on the diverter valve body started to strip when I tried to remove it .
Not sure how to proceed. Put a big pipe wrench on the diverter handle and try to force it? Grind the screw head off? Call a plumber :-(
There's no shut-off valve to the tub. And access to the pipes behind the shower is extremely difficult- up through the attic and then drop down behind the wall into a very narrow space that is filled eight feet deep with blown-in insulation!