If you discard a card but that card is not in your graveyard as Bag of Holding's first ability resolves, that card remains wherever it has moved.

You both draw and discard while Bag of Holding's second ability is resolving. No player may take any action—nor can anything else happen—until you've both drawn and discarded.

If Bag of Holding is moved to exile when you sacrifice it (most likely due to Leyline of the Void's effect), it remains in exile. It won't be returned to your hand.

If you control more than one Bag of Holding, you choose which one will hold the discarded card. Other Bags of Holding can't return that card.

If Bag of Holding leaves the battlefield, the items it contained are exiled forever (and, perhaps, scattered throughout the Astral Plane). If the same Bag of Holding card returns to the battlefield, it's considered a new object without access to the cards stored by the old object.