I doubt Ghirahim survived. The parallel between him and Fi should be obvious; as the spirits of swords, it stands to reason that they met with the similar fates and became dormant once their purpose was fulfilled. In addition, Ghirahim, like every other demon, disintegrated once his master was defeated.
The concept of purpose is a prominent theme of Skyward Sword; ever character plays at least two separate roles, and they all make sacrifices (except Demise, of course). The game is very bittersweet in that regard, though all of the main cast (including Ghirahim) are plainly content with their lot.
Ganondorf and Demise are the same being, that much is evident. If Demise's original spirit was destroyed over time is uncertain, though - it depends on how literally you take Impa's line. The game does not rule out the possibility of another sequel leading the way to Ocarina of Time, though I doubt we will ever actually see anything like that. In either case, Ganondorf certainly retains some traits of Demise's, but if those are residual elements of Demise's consciousness or merely aspects of the essence of evil is another matter.







