VintageMopar - I always welcome it when you chime in on a discussion. You invariably add clarity and insight,
The oil (in older coils) and epoxy-like material (in newer coils) serves to isolate and insulate the wires inside the coil. When the oil leaks out, the bare wires are allowed to come in contact, creating an internal short circuit. The same happens, to a lesser extent, when the 'epoxy' melts or disintegrates. In both cases, this is usually due to excessive heat. As you noted. the usual cause is internal heat due to the coil being subjected to extended periods of full battery voltage. Less frequently, this can be due to external heat 'baking' the coil.
You also mentioned the Mopar dual ballast resistor. In Mopars with single ballast resistors, the START circuit bypasses the ballast resistor, feeding full battery voltage to the coil. 0.5 ohms isn't a lot of resistance so there isn't a lot of difference between single and dual resistor systems. People risk damaging their ignition coils with either system when cranking the engine for extended periods. (More than a minute or two and without several minutes pause between starting attempts.)
Note: Until several months ago, I always thought the purpose of the ballast resistor was to save wear and tear on the spark plugs and distributor points. I now know the resistor primarily serves to prevent damage to the coil.
I also appreciate your comparison of GM, Ford, and Chrysler ignition systems. It helped me understand something I've noticed as far back as the early 1970s. Dodge (and International) trucks are notably easier to start in subfreezing and subzero temperatures while GM and Ford vehicles are much more difficult.
My mom used to get frustrated when her relatively new Chevrolet Impala was difficult to start, even though it was in an unheated garage with the block heater plugged in. She tended to get very annoyed when my '49 International Metro van would start right up even though it had no block heater and was parked in the open at the edge of the yard.
This was most notable when northern Minnesota temperatures plunged to minus 30 degrees F. My Metro would fire up on the first attempt, with only a second or two of cranking, while she had to make several attempts, each spanning a minute or more of cranking.
Even at severely subzero temperatures, I'd sit in the Metro while it warmed. I learned I'd have to listen to her pitch a fit if I went inside. (She didn't understand and hated the Metro, often saying, "Nobody would want to steal that thing.")
