I never figured out the actual cause of the problem, and others reported that replacing the body ECU doesn't always fix it, and nothing else is wrong to this day. But I have good news: I figured out a solution.
That purple ground wire mentioned earlier runs to pin 12 of junction 2F (bottom left plug) in the passenger fuse box. For whatever reason, like I mentioned, it's feeding 12v positive when it's supposed to be switched to ground by the body ECU. This seemed redundant to me. Why couldn't it just go straight to ground? All I did was cut that purple wire and send it to a new ground from the cable wrapping side. DO NOT GROUND THE END THAT COMES OUT OF THE PLUG or you will blow your ECU-B2 fuse. In the pic, you can see that I cut the purple wire and twist-spliced it onto a bit of green wire I had laying around. I hook that green wire to ground on a bolt somewhere above the glove box. TADA! Everything works perfect now. Easy fix, hopefully I can save you two weeks of prodding with a multimeter.