Yes it is a gremlin, but I've narrowed down where he lives! It seems when:
(1) the engine is disconnected from the wiring harness and (2) the points are open, the wires connected to the points are NOT grounded.
This is correct because if they were grounded, there would be no reason for the spark plug to spark to ground.
HOWEVER, when (1) the engine IS connectected to the wiring harness and (2) the points are open, the wires connected to the points ARE grounded. because of this, there is no reason for the spark plug to live up to its name.
Hopefully this faulty ground is an easy find.
God bless you multimeter, and thanks you guys for the advice and encouragement!