A token's owner is the player who created it, which may be different from the player who controls it or, in some weird cases, different than the player under whose control it entered the battlefield.

Neither ability cares who controlled the creature that died, only which graveyard it was put into. This means that if an opponent controls a creature you own, its death causes Magus of the Bridge's first ability to trigger, not its second.

Unlike its namesake, Bridge from Below, the Magus's abilities work only while it's on the battlefield. They do not work while it is in your graveyard.