I believe my 1964 \6 choke bimetallic spring is broken – Can anyone confirm?

no, that is the way they are made. at least the later ones.
something is just sticking

In the past, when I couldn't figure out what was sticking, I just Zip-tied a nut to the choke down-rod, which made it heavy enough to drop on it's own.

What you can try, which is where I found the stiction, is to disassemble and clean the fast-idle cam. If yours is plastic, the pivot bolt will corrode and eventually the cam will not rotate of it's own weight Just clean the screw, polish it a bit, reassemble, and try it.
Oiling it is a no-no, which will attract dirt.