My theory is this:
There exist infinite dimensions. However, the vast majority of them exist only in theory--they exist as possiblities. Chronopolis ghost says,
"Everything is purely a
problem of possibilities,
and the world is only
stabilized by the viewpoint
of the one who observes it."
Who does the observing? I posit that it's the entity/planet. That is, there're infinite universes, but they only exist when observed/interacted with by a planet's spirit. Since the entities are are the closest thing the Chronoverse has to gods, this makes a certain amount of sense. That might explain why everyone in the know can observe both dimensions--they're connected by a single entity's observation. That's also how Dinopolis came into existence--the planet, in a mighty effort, observed the Reptite dimension briefly and thus Dinopolis sprang into existence. The RD journal could just be an easter egg, or it could be an image from a defunct dimension, or even one animated by a seperate entity.
This also explains why time reflows--the Enitity watches Chrono & Co.'s journeys--and I'd imagine the Entity to be protected by Time Error, either by a Pocket Dimension or because dude, it's the Entity. It's powerful like that.
