1. No need to use a backup disk or USB, the Reset feature in Windows 8 will return your system to its factory state. Nevertheless, an extra backup image is always recommended in case the OEM restore partition gets corrupted.
2. On different brand computers the key to press to access the UEFI/BIOS firmware is different. From what I see on the 'net F2 is the key you should use. Also, even after setting the UEFI firmware to Legacy you still may not see an option to boot from an optical drive unless a disk is already in the tray. On some computers you can also press F12 of F11 to choose a device to boot from. You should enquire from Dell and find out what these keys are.