Unfortunately no, they didn't come back at the end. This seemed to be a chapter that the author wanted to include in the first volume, before "Medusa", but couldn't find a place for it, hence the "Lost Chapter".
Just the other night I began shouting "Don't let them see a photo of the milky way!", because our galaxy looks like a spiral. Funny to see it included somehow.
Kirie's brother, the last snail people, ended forever alone. 10/10