Can someone explain how the scenarios work? Why has the dragon played this scenario before? Do the scenarios repeat, and is this the 73rd time? Also, what exactly are the returnees?
Returnees are people who, at death, return to a previous point in their timeline.
IIUC this is splitting timelines, as the beast tamer girl from one of (subsequent? I don't remember) Joo's runs (as narrated in the book) happened in this run
The dragon has played a different instance of the demon realm game, which at this point is fair to think is made anew each time a new demon realm is created (not unlike a game of werewolf)
As for scenarios in general, I think that's up to the dokkaebi; all we know (or I remember) is that they are for the benefit of the constellations's entertainment, and span the universe: a bit like out TV shows and target demographic