Oh man, you cut it off grain on purpose?! That takes guts! Slippery fabrics are the worst for this, I feel your pain. I finally broke down and bought some pattern weights, those little metal washer ones, and they help a ton. It was a game changer after I messed up a whole yard of silk charmeuse, never again.

That part about everything testing fine until you load the circuit hits home. When you used the test light between the negative and the ground strap, were you basically making the light itself the load on that ground path? So a dim light means high resistance under that small load, right? How's that different from just checking for voltage drop with a multimeter while wiggling wires, is it just easier to see the change in the light?