Hi Toby,
Welcome to the Rise forums.
To answer your questions:
Q: Is it possible to export the source files for team access/back up?
A: Rise is entirely web-based, so there is no source file like Storyline to download.
Q: What happens to the source files if a persons license has lapsed or they leave?
If you have a personal type account, your courses will be saved for a period of 6 months after your account lapses.
If you have an organizational/teams account, your courses can be transferred to another team member if you leave the organization.
In order to maintain access to your editable Rise course, you need to maintain a paid account.