Got a 40 year old Delta 3-handle setup in a fiberglass tub/shower. For
the last 20+ years, the diverter valve has been left permanently in the
"up/shower" position and has frozen/corroded that way. Can't turn it at
all to run the water through the tub spigot.

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!