@Kuro
first off to clarify, the sword of darkness is not as old as Satoshi is in total. Secondly, as Satoshi is not human (at the time) the blade is not retained in any form by the king of heroes.
A: the weight of a life is a reference to feeling someone die, to watch the life drain out of them. Ask soldiers, it's a haunting terrible thing. Satoshi needs to absolutely feel the weight of Gilgamesh's life when he kills him. It's one of the few immovable character moments that exist within the story.
@Umbra
A: Canon says that while they will return it's time based, not like archer who can instantly retrieve the item, further some of the weapons were flat out destroyed.