I feel reluctant to post this here because I if it works, I don't want anyone else abusing this combo, but I need an official ruling, so I guess I have no choice.

If I play Final Destiny, and I have a Night Assaillant in my Hand, and one in the graveyard, can I just keep discarding night assaillant to ge tback night assaillant until I've done it 5 times? I would think so because each card must be discarded individually, and not all at the same time, correct?

No this will not work. The discarding of the cards is a cost and must be done immediately. With your logic, I can draw from Graceful Charity and discard the cards for it 3 turns later.

No, I'm saying each would have to be discarded individually. When you have a night assaillent with graceful charity, I know that his effect turns it into a pot of greed pretty much, so why would the same not apply here?

Night Assailant's effect does not resolve until AFTER Final Destiny has resolved. It's the same as if you try to use it for Ritual tributing. You can't discard Night Assailant, count the stars, get it back and repeat because its effect won't resolve until AFTER the Ritual is summoned.

So in this case, you would discard for Final Destiny, Destroy all the opponent's cards, THEN Night Assailant's effect would resolve.

Also, do not forget that you CANNOT bring back the very same Night Assailant you discarded either.