I’m sorry had to step out today also.

You wrote:
"Regarding your last idea, the flaw might be that in order to
get the coil to fire on the Stevens tester, or otherwise,
one side of the secondary must be grounded, therefore
I don’t see how you’d tell if both plugs fired at any given time.
Am I thinking wrong here?"

When the flywheel spins in real time it grounds one side of secondary automatically if brushes and flywheel tab and no cracks in bakelite and its all working.

That is why I was thinking you could put your flywheel in the position on engine and where coil would normally fire that should kill one cylinder fire if brushes and tab are working. Snake a wire and check with stevens meter.

By doing so your checking both plug wires with stevens meter to see if the other side of coil secondary is grounding out as it should with bakelite brass piece in flywheel . The brass tab did show a correct ground when you bench checked flywheel with omc coil and made sure tab was grounded and no cracks in bakelite. You did get spark with the probe when you got near brass tab but no spark checking bakelite thats a good thing. it means Good Ground on tab and good bakelite no cracks?? Hope that makes since.

One side of secondary should automatically be grounded when coil fires by flywheel brass tab and brushes are is working correctly which I suspect it isn’t. You have 2 lobes on cam so when one cylinder fires one shouldn’t because brushes. When cylinder fires one shouldn’t because other brush and by shouldn’t I mean they can’t or motor will never run right.

One more test to possibly try. Use stevens meter to fire your coil and see if both spark plug wires put out spark with flywheel off.
By checking one plug wire for spark then the other with flywheel off. I bet you a bunch of money that they both fire when one is not grounded and that will help explain the need for the brushes and how important they are for correct ignition results. What kills one side in real time is the brushes grounding one side or doesn’t kill the other side then you get double firing which I think you may have.

A zephyr has 4 cylinders 2 coils which means 1 coil has to fire 2 plugs/cylinders at same time but one is one piston is in the tdc position and one in the bottom dead center. Since their is 2 coils on a zephyr 4 cylinders more than likely 2 lobes they alternate all that and don’t need the bakelite contraptaction you have in your flywheel.

Check your capacitor for leakage also. Try a good tested condenser with good leakage reuslts. The motor does also sound like a capacitor that may be breaking down. With one side of coil grounded everytime it fires I would guess a good condenser/capacitor is even more critical. It really sounds to me like an ignition issue.

If you can’t get it worked out we might try one of my home made red atoms in it since you have a type of distributor using the flywheel and brass tab. I have only had success with the omc engines 2 cylinders 1950-1960ish but both magnets are on one-side of flywheel your magnets are on both sides of flywheel but with the flywheel used for distributor who knows might work. I have heard folks have used them in Zephyrs and they also have multiple magnets.

I hope that helps,

Joe