To turn OFF iPhone you can use program Tiny Umbrella. To turn iPhone back ON you use Tiny Umbrella again. All very simple actually.
Download Tiny Umbrella to computer, open iTunes and Tiny Umbrella, connect iPhone to computer with USB cable. Select iPhone name in Tiny Umbrella, click "Enter Recovery" button in Tiny Umbrella. When you see the iTunes Logo on iPhone screen it is in Recovery Mode. Now disconnect iPhone from USB cable. 30 seconds later iPhone will automatically turn OFF.
When you want to turn iPhone back ON, push Power button, or in your case if Power button fails, just connect iPhone to USB cable and computer or Wall Charger. iPhone will power right up, but will still be in Recovery Mode. Connect to Tiny Umbrella on computer, select iPhone name, click "Exit Recovery" button. Done. Actually very easy once you give it a try.