I found that the damage to the primary harness (93 500E) was most easily demonstrated/diagnosed by going to the injectors and sliding the boot up and off the injector cap. This exposes the wires to view, and in this case, they were thoroughly cracked.
The "crumbled to dust" was not in evidence except for one section where the outer jacket (the stuff which Andy has cut away in the first photo of this thread) was torn. Inside that tear, the inner insulation wasn't cracked, it was entirely absent. Two bare wires.
I would hypothesize that the jacket provides some protection where it's intact, but once the jacket gives and exposes the cracking of of the insulation inside, the inner insulation powders away quickly.