1) Themes are sold separately, however I am unsure if there could be packages available in future which may allow purchase of multiple themes? You may wish to wait for Septemper to see if this happens but I can check with the developer if there is any chance this might happen.
2) I would have to check this with the developer.
3) Yes you can customize the theme as you wish, and your best option would be to create a child theme, as this will make the upgrade process a lot easier.
4) This is a customization you would have to make yourself, as it's not how the expiry notification currently works at present. So really it comes down to your coding knowledge whether or not you can get it to work how you want it to. You would also be welcome to submit your suggestion as a feature request for consideration for a future release of CP.