You can go back to Sablee and Auria by using the pod in the very top right of the Rietti plains! It only appears after you beat the demo.
There are 13 power orbs in total! There is a secret in the bottom right room of the top floor where you have undiscovered rooms ^^
I also have a walkthrough here if you need it: https://www.patreon.com/posts/119207938