cgmsys writes:

In the meantime, I'd like to seal up the coping stones to keep them
from deteriorating any further. I can see a couple of options and
would like any opinions.

Just get some sanded grout, make a dry mix, and pack it into the voids.
You can use unsanded grout as a skim layer to cover rough areas. Use an
polymer modified type to get the best adhesion and workability. Very
laborious so not practical except for limited areas of attention.

The Quikrete acrylic fortifier would make a fine, clear, paintable sealer
right out of the jug. You don't need "concrete" paint. Any outdoor
acrylic would do.

Forget Thompson's. That's just wax in mineral spirits.

If you want to make a serious effort of it, you can make your own mold to
duplicate the existing cast coping stones. It's not that difficult to make
concrete molds on a hobby basis. White portland cement is at Home Depot,
mix with gravel and quartz sand to make your own white concrete.

Concrete should last forever. Not clear why your coping is disintegrating.