So, how does Roxy's session create Caliborn's universe if it's void? Is that what we might end up seeing because of the post-retcon addition of the beta kids, who might make the session no-longer-void?

A Void session means that the players don’t prototype anything pre-entry and their Battlefield never evolves and grows. Jade has the Beta kids’ fully prototyped Battlefield so they just need to place it at the center of Skaia.