I'm sure the disaster in the first story was a series of meteorites
Or boulders that fell from eruption? You never know when ending is abrupt and author changed his idea and morphed it into another one. You know how its like "oh well that ending wasn't exactly what happend, actually..."
Maybe I'm wrong but with that happening all over the place it became a habbit expecting something like that.
Just a side note, a volcano eruption can send (big) stones flying pretty far.
"I'm your very own life force!" Sure... go back to your corner «Joker», it's not funny.
And that Moro(n?) who can say "I'm not good at doing things as a group" with straight face holding a K
What Gamon tried to say until he was interrupted (since he already started) "Basically this world is our-"... Final Road, the title «Imawa no Michi» of this manga.
From what Gamon recalls it seems there was 5 people who volunteered for this and thus were made chosen side by their original intention and Jokers regulation of the Borderworlds, it kinda synergized. And those 5 had personal reasons to attempt this it seems, at least 3 of them have identifiable motivation(Kimari,Gamon,Moro). I wont be surprised if other participants come from nearby clinics being in near death state and the 5 interfered with their journey.
And connection to the first story is that the disaster that caused people to go "borderlands" was strong Volcano eruption or so it seems.