Where the AC mains romex goes through the hole in the 2x4 across the
top of the studs and on up into the attic, there is some type of
hardened reddish gunk placed around the cabling to plug the hole in
the 2x4 and hold the cable rigidly in place.

My guess is that the purpose of the 'gunk' is to hold the cable,
prevent critter intrusions, and provide insulation between hollow of
interior wall and attic.

What is that compound?

Also, is there a similar, but more pliant, sealant? Just in case the
cabling needs to be replaced and the pliant sealant will allow
everything to be pulled out?

*It is a firestop caulk. The color denotes the rating. I think red is
rated for 3 hours. They sell them in tubes at Home Depot where the caulk
is.

Thank you for identifying Home Depot. IBut it would be nice to have
the copound a bit more flexible. That red stuff is like a rock set in
there.


*Duct seal (Also available at Home Depot in the electrical department) is
more pliable and remains that way. However I don't know if it is fire
rated. Duct seal is usually used to prevent infiltration of air or water.
Maybe if you packed the hole with mineral wool and then used duct seal it
would give you a decent fire stop.