Marchesa, the Black Rose

If the creature card leaves the graveyard before the delayed triggered ability resolves, that card won't return to the battlefield, even if it's back in the graveyard when the delayed triggered ability resolves.

If a creature has multiple instances of dethrone, each triggers separately.

Once dethrone triggers, it doesn't matter what happens to the players' life totals before the ability resolves. You'll put a +1/+1 counter on the creature even if the defending player doesn't have the most life as the ability resolves.