While Garland is on the battlefield, creatures you control but don't own can't be sacrificed for any reason. If an effect instructs you to sacrifice one of them, you can't and it remains on the battlefield. You also can't sacrifice one of them to pay a cost that requires you to sacrifice a creature.

If an effect instructs you to sacrifice a creature and you control any creatures other than ones you control but don't own, you must sacrifice one of those other creatures. You can't try to sacrifice a creature you control but don't own.

If the monarch leaves the game during another player's turn, that player becomes the monarch. If the monarch leaves the game during their turn, the next player in turn order becomes the monarch.

The game starts with no monarch. As a player becomes the monarch, the current monarch (if any) ceases being the monarch. There is never more than one monarch at a time.

If combat damage dealt to the monarch causes that player to lose the game, the triggered ability that causes the controller of the attacking creature to become the monarch doesn't resolve. In most cases, the controller of the attacking creature will still become the monarch as it is likely their turn.

The owner of a token is the player who created that token or, in the case of a resolving copy of a permanent spell that became a token, the player who controlled that spell as it resolved.

While Garland's second ability is on the stack, if that opponent stops being the monarch before it resolves, you won't gain control of the target creature at all.

There are two inherent triggered abilities associated with being the monarch. These triggered abilities have no source and are controlled by the player who was the monarch at the time the abilities triggered. The full texts of these abilities are "At the beginning of the monarch's end step, that player draws a card" and "Whenever a creature deals combat damage to the monarch, its controller becomes the monarch."

If the triggered ability that causes the monarch to draw a card goes on the stack and a different player becomes the monarch before that ability resolves, the first player will still draw the card.