I’m very grateful for this website and its invaluable photos and instructions.
Just to say: I was able to change out a defective audio port cable and socket set without having to remove everything detailed above. I was able to follow the steps on this page:
iMac Intel 21.5" EMC 2428 Audio Ports Replacement
and, with a bit of effort and small hands, get the new cable plugged in behind the fascia. It’s all working again now.