I'm hesitant about jumping into a well-oiled discussion.
SiN's right about isolating. Put the parts next to a working box and carefully swap junk until you find the culprit.
Specifics that come to mind, simple stuff first:
- Standard (non-USB) keyboard & mouse attached? I've seen too many low-end boards that didn't seem to include headless operation in their test suites.
- Different video card?
- The 24->20pin power supply adapter is a failure point. Do you have another PC you can sit on the bench to confirm the supply/adapter isn't a factor?
- Don't worry about the fans running during your lights-on-nobody-home tests. It doesn't point to anything specific beyond that the basics of your p/s are working. Also, don't let it just run with the clear CMOS jumper in position. Depending on mb design, continual power with the jumper can harm some components. When clearing, use the minimum jumper duration required to restore the POST.
- Don't worry about flashing the BIOS. You're right, it implies getting further than you have.
- The "bent pin" processor experience sounds ominous. Could be terrible or not a big deal. Can't know from here. If you're confident then don't mess with it until you're out of options. Still, given the number of other things you've tried, it would be haunting me for a test in a known-good box.
- Have you tried with the motherboard removed from the chassis? (on a non-conductive of course)
BTW, you're discharging the p/s before installing or removing components, yes? It's almost difficult to hurt things this way these days but I hate to see someone repeatedly blow away parts.
Crap, I just had a copy/paste moment. Think it's all back now...
From this point I resort to my remote-trouble-shooting voodoo dance. Not pretty.