In Cascade, where were they when the Tumor exploded? Like, I know that Rose and Dave were in Derse, but were they in the furthest ring? If so, then why was their Genesis Frog sitting there? Wouldn't wherever the Green Sun was created be inside the Genesis Frog?

Dave cut the chain connecting Derse’s moon to the planet and they traveled in the moon to the Furthest Ring. The Furthest Ring and the Medium are separate planes from the Genesis Frog.