We are here to help you. I understand your issue.
The purpose of the registration system is to make sure 1 valid installation is in the place at the same time. But it does not limit you where you want to install it.
So you can install the theme on your staging server wherever you want including local machine or subdomain.
Whenever you want you can go to this URL and delete the existing API Key and generate a new one for your another server:
It will invalidate the current installation and now you can re-register your new server again with new API Key generated.