Their is one other thing that you can try to do, if you put the imac on a table, screen facing down on a towel or something, you should open the bay where the memory goes, on the bottom left side, look inside the imac with a flashlight, you might see a lila/red colored button, thats the PMU reset button, press it when the unit is OF the power, another thing that crosses my mind and what you can try is resetting the SMC of your imac. Unplug the power, keyboard and mous, press the power button for about 5 seconds, then plug everything back in and see if the imac will boot, if it doesnt, its more likely to have a power supply/analog board faillure due to age. Good luck with your lovely imac :)