That's correct, only for PS4 is that not possible as we don't have the ability to take saves off of their cloud and vis versa (it's a security issue with Sony)
That's correct. You would just order another server. Once you have both servers you can link them together by specifying the Cluster Code that matches:
It would keep your saved world (so buildings, tamed tinos, etc), not player data though.
You can take a backup if you'd like or revert to a backup from the drop down which will retain player data but those backups only last a few weeks. So if you are constantly switching back and forth between maps within those few weeks it's doable with the backup restore feature. If it's going to be longer than the listed backups you would need to make a manual backup of your SavedArks via FTP. That folder location is:
ShooterGame\Saved\SavedArk*
You would just go to the My Account page and cancel any automatic payment. When the next invoice is generated you can select a credit card or create a new PayPal subscription.