Just to give an update: I'm not getting anywhere
This morning, I removed the mb again, and took out all extra standoffs, going back to only using the 6 needed to match the screwholes. I had also cleaned off the most the excess heatsink compound and respread it. I took out the single dimm I had in the box to listen for memory beep codes. Nada. Reset the cmos again (including taking out the battery), and same behavior.
Still, with the 4pin 12v connector attached, I get no activity beyond the standby LED. With the 4pin disconnected, the cpu fan will spin and run for about 15 seconds before shutting off. No beeps either way.
This is with only the following config:
cpu installed, and cpu fan power attached
24pin power connector attached
4pin 12v power connector both attached and unattached
no dimms
no expansion cards of any sort
no power running to any other device
no keyboard or anything external beyond the power cord
/edit: Does anyone know what exactly that 4pin 12v power governs? Like does it primarily power the cpu, or does it primarily power expansion slots, or.. ? Perhaps if I knew exactly what it did besides provide extra power to the board for stability, I would have more of an idea of what to look for.