Keep us posted on what you find. Note that you can remove all the electronics from the IMA bay and the car will still start (as long as charge remains in the 12 volt battery).
After ruling out pretty much everything else, I bought a new ECM at a local junkyard. When I pulled the old one, this problem at least was immediately obvious:
Pins 1 (STC), 2 (ENGRDY), and 12 (IMOLMP) on ECM Connector A all had significant corrosion. I do remember reading a thread where someone had a similar problem at one point, so this is apparently a thing. I sincerely doubt it was the cause of my current problem since that corrosion has probably been there a good long while, but it does explain why sometimes the relay used to click a few times before the car would start even though I had replaced all ground wires and thoroughly cleaned all contacts.
The STC pin is what closes the starter cut relay so your car can start. Even though I don't believe this was the primary cause of my current complete inability to start, the clicking was annoying, and I'm sure if the corrosion were bad enough it would have kept me from starting the car altogether.. If anyone is having starting problems, you may want to visually inspect your ECU pins, because I'm at least the 2nd guy that had this problem.
I couldn't see any evidence of damage on the board, but there are a lot of tiny components on there, and the damage from an overvoltage event might not be visible anyways.
So while I was putting my new ECM in, I managed to set off the car alarm somehow. I think something must have been busted in the old ECM because in the 8.5 years and ~140,000 miles I've had this thing, I've never had that car alarm go off once. I've even pressed the panic button and gotten nothing.
Anyways, I rushed to turn the car on to cancel the alarm. Decided to try starting the car while I was there, and I got nothing. The ECM wasn't properly secured yet, but everything was plugged in, so it should have been OK, right? Except I forgot about the immobilizer. Correct me if I'm wrong, but the key is paired with the ECU, right? So swapping in a new ECU means I need to either desolder and swap the immobilizer chip, or add on an immobilizer bypass chip. Is that about right?
Lol, this has been a headache, but I totally brought this on myself. Always double-check your wiring, kids.