Not for nothing, but my 24-inch mid 2007 iMac also went completely defunct in one power cycle. I actually brought it to an Apple Store where they replaced (according to the paperwork) power supply x2, backplane, and main logic board. They ended up returning it to me "no charge" and pulled the hard drive at my request. I'm following up to ask you if you have had success with your repair. I'm leaning toward replacement but if this problem turned out to be something simple and mechanical like the power switch, it would be totally worth my time to tear it down and repair it.
Slow and easy seems to be the key to separating the ribbon cable from the top of the drive. I used an alternating approach, emphasizing first one side and then the other of the cable, especially around the curve. The spudger and a plastic card were both useful here.
If you are using a muffin pan to keep removed parts separate (a method I recommend highly), note that Steps 5, 6, and 7 involve removing the same size screw—and thus you can put them all in the same pile. This is unlike just about every other part you will remove during this procedure.