>I even took the coils off and flashed each one direct to the battery - no spark,
Mr. Thrasher is correct, that isn't how real CDI charges the coil(s).
>After a lot of testing / double checking I found the HT windings on the coils were reading between 20 Mohms and intermittently infinity.
Might be relevant but, being HT, might not. What are the coils' the LT resistances (I expect low so you'll need access to a good Ohm-meter for accurate readings)?
>Iridium plugs have a larger gap
Basically, just means the coils have to generate higher HT Volts to jump the gap. Worst-case, the HT could've found a path to earth through old insulation but seems unlikely in three separate coils at the same time ...?
Afaict, iridiums most commonly-available have a 5 KOhm resistor; if the replaced plugs didn't, that's extra resistance in an old HT circuit, which isn't ever a sensible idea.
New plugs with resistors, have you considered they've failed?