I like the more introspective chapters like this. The theory-crafting by the characters as they attempt to navigate alien phenomena is engaging. It’s a whodunnit but with a phenomenon instead of a crime, and is something we seldom see in any narrative media. Honestly at this point it’s the closest thing to Steins;Gate’s theorising than anything I’ve seen, though because Urasekai Picnic isn’t really describing an objective reality, I wouldn’t call it engineering fiction.
This, coupled with the unnerving confrontations and mental shock, especially the retroactive shocks like talking about the phone call with Kozakura from the train station, make this such a well-crafted story.
Give it a chapter, I bet at least one of them brought a gun with them, which they’re doubles going to need. My guess is the two get whisked to the otherside and leave Kozakura behind, who has to travel back home without them. Dragging Kozakura to the otherside would just be too mean, but you never know.