OK maybe I can help you...a bit. Disk Utility should be on your mac laptop's hard drive under the "Utilities" folder. I don't know anything about giving your laptop back to Apple. None of us work for Apple and Apple doesn't post here. We're just Mac users, same as you. Anyways, try running Disk Utility from there, and as for getting into the Recovery Partition, it's command/cloverleaf/Apple key and R held down together. If you have a windows keyboard, it's the "Alt". As for the serial number not recognized, I don't know about that either. You'd have to talk to someone else about that. IF you're absolutely desperate to erase your hard drive, and you've also done a backup, and de-authorized it for the App store and iTunes, and I'm guessing you have, which is good...then If your laptop is old enough, you can buy and use a dvd copy of Snow Leopard, boot up from that and run Disk Utility. Please note that in order to find Disk Utility these days, you have to move the mouse to the top center of the screen, OS X Tools will show up, and from there you can get to Disk Utility
hope this helps you some
john B