Holy cow... what a ride. I think I've finally got it sorted... I haven't done a road test yet, but it passes all the diagnostic tests several times, with a couple stomps of the pedal in between repetitions.
Edit to add: Road tested, and works as normal.
So with the new switch (napa house brand), I was reading 12v+ at terminals 1,5, and 6. Oddly enough, i was reading an open circuit 5-6 with the body harness unplugged. I opened it up and the cc switch had about 0.005" of opening - open enough to not arc with the millivolt test signal from my multimeter, but close enough to arc with 12v.
I also noticed that with the original switch, i was reading 12v+ at 1, 5, and 6, but a closed circuit 5-6. Then i noticed the rattling from the switch.
Turns out the factory switch has a 3rd switch internal to the switch body, that doesn't connect to anything on the body harness. This 3rd switch had broken, and a fragment had fallen down onto the cc switch, shorting it out.
Tl;dr: aftermarket switch was bad out of the box, factory switch was repairable. Cruise works... for now.