I am actually having a feeling how this story plays out, these 12 Lives arent his lives, like not in the world he lives in. Death uses these 12 Lives to let him fear death and reconsider in what he thinks is his world. But in actually its like some parallel Universe bs or virtual reality. After all 12 Deaths he vents how he feels and maybe how scared he is. And then later Death just goes "okay you've done well" and he ends up waking up in a hospital since he hasnt actually died.