So this turned out to be the most hilarious car problem I've ever had.
After all this ****--after doing the valve blocks, checking the wiring, taking the car to the dealer to have them look at it, having the dealer tell me the exhaust valve on the compressor was bad and I needed a new compressor, and ordering a new compressor 2-day air so I'd have it in time for this weekend--I pulled the compressor on my car just now and found the problem.
I sat there with my compressor in my lap and started tracing the wiring from the plug that you can see underneath the compressor when it's installed to the exhaust valve that you can't see when it's installed. Didn't see anything bad until right before the exhaust valve, where I saw that the wiring was legit just fully unplugged from the exhaust valve.
"Surely this can't be it?" My first thought was that maybe the dealer had unplugged it to run some tests and just left it unplugged before they reinstalled the compressor, but then I thought I only paid for an hour of service, so there's no way they pulled this thing, ran tests, then reinstalled it.
Plugged in the wiring clip, reinstalled the compressor, and everything works fine now.
TL;DR: the wiring to the exhaust valve was unplugged.
If you want to know what wiring connection I'm talking about, the exhaust valve is the part right above the "H.c" in the watermark in this photo, and the wiring connection that was unplugged on mine was the wiring leading to it:
No idea how it came unplugged. I guess it wasn't fully plugged in when it was installed in August 2018, and took this long to work its way unplugged? Who knows. Imma going to go grab a taco.