If you've tried all of the tips in the link, honestly, I can't for the life of me think of any ways to further test the system.
However, when power supplies fail, more often than not, they will still power the fans. You mentioned that you powered the computer off and, I assume, left it for several days. Some power supplies may be adversely effected by being left powered off if they have a defective component.
The PowerMac G4 MDD was notorious because of bad capacitors in the power supply. Leaving the PowerMac plugged in but not turned on would heat the capacitors and eventually bust them. Your iMac's power supply may have had a defective component. It happens fairly often.