FINALLY figured out the wiring.
Not sure I had a picture of this before, but there used to be a three-gang switchbox that had 6 romex lines coming into it in the corner of my garage. 3 way circuit for the overhead lights, 3 way circuit for the front exterior lights and a 3 way circuit for a light inside the old room we tore out. I was able to remove the 3 way circuit for the old room because I had both switches and all the wiring exposed, but could not for the life of me figure out how the other two were wired.
For the exterior lights, there was a 3 way switch in the dining room that worked with the 3 way switch on the exterior wall of the garage and two different romex runs came into the garage from the dining room. After some research and some swift multimeter work we were able to convert the interior switch to two-way and cut out the second switch in the garage. Turns out the lights on either side of the stoop are fed by a line that comes into the house from the garage, who would have thought. WIN!!
For the 3 way overhead light circuit, we beat our heads against the wall for quite a while. We cut out the second switch and wired the new outlet and single two-way switch circuit up correctly, but every time we put a load on the circuit it would trip a GFCI outlet in the garage that shared the circuit. A load plugged directly into the GFCI did not trip it, but that same load plugged into our new socket did. We opened every light socket and outlet past the GFCI outlet to make sure everything was wired correctly and it was. We used a GFCI circuit tester and it indicated proper wiring not only on the GFCI outlet but also on our new outlet, so we were confused. Finally, we pulled the GFCI outlet and found that the load side hot wires were correctly connected to the load terminals on the GFCI, but one of the load side neutral wires was connected to a line terminal on the GFCI!! ****!! Someone must have done that due to the messed up 3-way switch circuits that were put in. Finally, after fixing that incorrectly (for our new circuit plan at least) placed neutral wire, everything worked as expected and tested properly with the GFCI outlet tester :cheers:

:grinpimp:
The reason we did all of this was to put an outlet in for the parts washer and put the garage overhead lights on a single switch so we could install a new light I picked up. Here's the new outlet and light switch. Garage circuit feeds the outlet, then the switch is fed by the outlet, which switches the two overhead light runs. Here's the parts washer outlet that's always on when the breaker is on:
We repurposed an old wire run for what was presumably overhead lights in the back half of the garage back when the garage was built for an outlet off of the switch. Here is the new light mounted and plugged into said outlet:
This is a new light by Big *** Lights, the same company that does Big *** Fans. This is their "Garage Light" and puts out 13,000 lumens. It's blindingly bright, so much so that they even package a pair of cheap sunglasses with the installation hardware

The light is expensive at ~$400 shipped but it replaces a few T8 fixtures converted to LEDs based on the output, so to light a whole garage with LED, this is cheaper than T8s with LED bulbs believe it or not. Plenty of threads over in garage journal discuss lighting cost analysis if you're interested.
The effect is not as dramatic in the day as at night, but this should give you an idea. FWIW, now that this light is in place, the rest of my garage feels like a damn cave because it's so dark by comparison. I used to think I had OK lighting but it turns out my lighting before was garbage.
Light off:
Light on:
I'll be purchasing a few more of these babies to put up around the shop when I can afford them. I'm very satisfied.
Aside from a lot more drywall work to close in and insulate the ceiling, this project is officially complete

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