Higgins wrote:
I can turn the water on and off (well, not completely off), but I
can't remove the handle to try and fix the leak. My father in law (an
HVAC contractor) banged on it and presumes 20 years of calcium
deposits are freezing the handle in place. He suggests liberally and
repeatedly spraying WD-40 in the screwhole and behind the handle over
several days. But I'm thinking I should be using something more likely
to dissolve the deposits. Any guidance??? Thanks.

Get the proper tool. It's called a "handle puller" (like a gear
puller) and works almost every time.
It's tough to get anything in there to dissolve corrosion.
Carefully applying heat from a small torch may work too.

Jim