I believe I am finally done exorcising my electrical demons ... figured out my intermittent horn issue today.
1) Started by directly putting 12v into my horn with it muffled in a big ball of blankets for quite a few minutes, to make sure the horn wasn't the problem (it was not)
2) Crossed the contacts of my horn switch with a screwdriver to make sure it wasn't the switch (it was not)
3) Disconnected the entirety of my harness from the front bulkhead onward, stripped the tape off the relevant parts, and double-checked all the wiring there (all was good)
4) Drilled out the rivets on the front two sections of my tunnel and disassembled the shifter to expose a part of the harness that I know had a solder point in the horn wire from earlier in my build (it was fine)
5) Running out of ideas, I took the fuse box lid off and tested continuity from end to end for the green horn wire (it was good)
6) Visually inspected the horn fuse, and then tested continuity on its contacts (it was fine)
7) Finally, I was reminded of my original electrical issue where the car wouldn't crank ... problem there had been a slightly damaged contact inside the engine connector, inside the fuse box. I took the headlight harness connector out of the fuse box and checked the horn pin there (that WAS the problem)
Didn't have it in me to rebuild the car after all that, so I'll do it one night this week, or this weekend.
On the plus side, I've all but confirmed I can get insurance through an agent that works with American Modern for $460 a year, $20k agreed-upon value, 6k miles a year. Just have to get registered first.