Problem found! So I figured I'd circle back on this old thread of mine.
To recap: My "kill" switch wasn't killing....there's still sparking on one of the cylinders. Many of you said "wiring issue" and I, perhaps a bit stubbornly, said that everything "rang out" with my meter, so I discounted the advice given. But it was true: My meter said that everything was fine.
Yesterday was the first warm spring day we've had all year, and even though the kill switch wasn't resolved, I was eager to bucket test the engine to see if my engine building efforts have been worthwhile. So, bucket full of water, I fired it up. It started & ran pretty good, but not great.....and it seemed a wee little bit "shaky". Not smooth. (And the kill switch didn't kill the engine, either)
Fuel supply seemed good. Air / carb...good. Whatever was causing the rough running had to be in the ignition. So for the umpteenth time, I pulled the flywheel and immediately had a "Eureka" moment: I had routed the kill wire on the top-cylinder points in such a way that it was making contact with the magnet hubs of the flywheel, and the flywheel had rubbed through the insulation. The exposed copper wire was making & breaking contact with the flywheel as it spun, grounding out & simulating point closure at the wrong time.
Today I replaced both kill wires, re-installed the points, caps & coils, and reassembled the whole thing. (Re)set the timing and reinstalled the flywheel & recoil. Everything rings out, sparks are strong and the kill switch now works properly.
Lessons learned: 1.) continuity on a meter doesn't mean "everything is right" and 2.) don't be so arrogant to believe that everything is right, especially when others repeatedly tell me "you have a wiring issue".
Now I just need another warm day to do another bucket test. I'm pretty sure this baby's gonna hum!