Hello Niall,
Thanks for writing in!
When you revoke the license, the existing theme installed in your site will still work. It will not crash whatsoever. Yes, it will be accessible and will work like a normal site. The only thing missing is that you cannot install any new bundled plugins and will not be able to update the theme, Cornerstone plugin along with the bundled plugins which you may already have installed.
And yes, you can use the license code to validate both the staging site and the live site simultaneously. You can also assign both url to your license code in the APEX (https://theme.co/apex/licenses).
Hope this helps.