good fluff explenation, however, they don't always inhabit metal bodies. in their natural forums they are simply increadibly powerful beings of purse energy who devour stars to become stronger. however, stars are pretty much just bread and water to them but the souls of living beings are a delicasy. the enslaved the necrotyre race and made them into the necrons, giving them undying metal bodies to live in. when fighting on the battlefield the C'tan will enter necrodermi to fight in. as said above sometimes the necrodermi are destroyed and the C'tan is released but since the necrodermis was only a shell the C'tan can enter another one when they want. I am not really sure why the C'tan need bodies in the first place as they were pretty devestating in their normal forms, i'll check up on that.
and one thing you are wrong about foxfire is that the ones awake are NOT the weakest. the nightbringer was the strongest of all the C'tan, the deceiver, (who is weaker, but not the weakest) is a sneaky smacktard and tricked him into devouring the other C'tan. this is found in the codex under the backstories of the two C'tan.
The reason they sealed themselves away is because they were afraid of running out of food. the galaxy was so devestated by them that they decided to let it regrow while they slept so when they awoke they could feast again. They were never actually defeated, they were just worried about their next meals. HOWEVER, the races they had fed on were tipped off about them going to sleep and ambushed them so only 4 escaped, and yes they were weakened (maybe that is what foxfire meant).
but i suppose it shows how strong the C'tan are when in their eyes the galaxy is not some place to conquer, but a snack bar.