Okay, following up, as promised:
Took vehicle to the nearest dealership with a written symptom complaint sheet (telling them exactly where to look). Received a call from the S/A that afternoon telling me that they concurred in the failing-harness diagnosis and that they would perform a full replacement. Tech was asked to inspect the other underhood wiring; reported no problems.
The new harness apparently had to be overnighted in (from Germany?), so it was a two-day job. When picked up, another S/A said that the work was "fully covered", but didn't specify whether it was Starmark coverage or a goodwill replacement. I suspect that anything that -does- have Starmark just clocks in under that policy. Fewer hoops for the S/A to jump through on the paperwork.
Now, with the replacement harness, the car is better than ever, and it was pretty amazing to start with. I suspect that it had been erratic on firing one injector, as the wires were really bad in that single case. Everything else had extensive cracking, but it was being held together by the jacketing around the wire pairs.
We've learned an important lesson today: inspect your harnesses! If it happened on this vehicle, it can happen on any car from the vulnerable production runs. This is not a neglected beater car. It's one of the best-kept 500Es I have ever seen, dealer serviced since new, and had passed a Starmark inspection in the low forties, not all that long ago.
But if I hadn't looked closely, I would have missed the cracking. It's that subtle. Most of it is hidden underneath the harness jacketing and the injector boots. I found that pulling the boots off was the fastest and least invasive way to scope out the problem.
Impressive that the car still started and ran quite well with the wires as messed up as they were. Now, with that fixed, hopefully the car is okay for the rest of its lifetime, and given the overall solidity of everything else, that should be a good long time.