First off: Yes you can use the spare power leads.
Second: the wire from the motherboard to the cd drive, no it doesn't "need" to be plugged in but you wont be able to hear any sounds ie music or speech from the cd in the drive. The solution: unplug it and buy a longer one from your local computer shop.
Third: The cable from the hard drive to the motherboard, yes you can use the spare plug half way up the lead, but make sure you set the jumpers on the cd drive to "slave"
and........you shouldn't need mounting brackets, but if you do, pick them up at the shop when you buy your longer audio lead.