Staggershock

If a replacement effect would cause a spell with rebound that you cast from your hand to be put somewhere else instead of your graveyard (such as Leyline of the Void might), you choose whether to apply the rebound effect or the other effect as the spell resolves.

If you are unable to cast a card from exile this way, or you choose not to, nothing happens when the delayed triggered ability resolves. The card remains exiled for the rest of the game, and you won't get another chance to cast the card. The same is true if the ability is countered (due to Stifle, perhaps).

At the beginning of your upkeep, all delayed triggered abilities created by rebound effects trigger. You may handle them in any order. If you want to cast a card this way, you do so as part of the resolution of its delayed triggered ability. Timing restrictions based on the card's type (if it's a sorcery) are ignored. Other restrictions are not (such as the one from Rule of Law).

Rebound will have no effect on copies of spells because you don't cast them from your hand.

If you cast a spell with rebound from your hand and it resolves, it isn't put into your graveyard. Rather, it's exiled directly from the stack. Effects that care about cards being put into your graveyard won't do anything.

If a spell with rebound that you cast from your hand doesn't resolve for any reason (due being countered by a spell like Cancel, or because all of its targets are illegal), rebound has no effect. The spell is simply put into your graveyard. You won't get to cast it again next turn.