Those figures look okay, but it's through the logic board where it's losing voltage and that's probably being caused by failing capacitors.
There's a couple of other things you could try -
1/ Plug the HD into a good known PSU (not the Mac PSU) and see if that spins the HD up sufficiently to boot. Obviously you'll need to have the HD data cable connected to the Mac logic board.
2/ Heat up the board with a hairdryer and see if you can get the capacitors sufficiently warm to enable at least one successful boot.