FWIW, do you know if the engine wiring harness has ever been replaced?
These cars have had trouble with engine wiring harnesses, and when you move the wires around, sometimes it causes shorts in the harness. If you can look at the insulation, if you see it flaking off, or if the insulation has turned powdery or is crumbling, or if it looks like its 2,000 years old, then I think you're going to have to replace it.
Also, it's not usually the spark plug wires that go bad, but the resistor ends. Do you have the resistor ends properly attached to the wires?
Just a thought.
Oh, almost forgot. Have you tried posting in the Hot Rods forum here? That is where most of the 500E owners hang out, and you'll find pages and pages of tech stuff in this section.
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".