Congrats, and thanks for closing the loop on the whole story.
I get a kick out of guys working on electric cars that consider 300v as really high voltage, they have nothing on ignition systems. Ignition HV, as you learned, is an entirely different ballpark. Boots are a requirement at our scales, ridges on spark plug towers, and slopes in electrode insulators, are all there to increase the length of the path a spark must travel to the wrong place. Under some conditions you can get away without them, but it can be frustrating finding out where that spark goes when its not where you want it.