I would try replacing the suspect coil first.
I would think the rest of the loom wiring is okay if the original top coil worked when you swapped it to the lower position.
Sometimes electrical components can break down under load. They can test okay for continuity, then fail in use.
Also it's harder for a spark to jump under compression. Putting more load on the ignition system than when the plug is just layed on the cylinder head.