Yes. User with enough permissions at some folder should able to use the import tools to import any content the user has permissions to add normally.
Yes. Pull for Plone documentation repository would be preferred.
Yes. That's a built-in feature of Plone 5 and beyond.
Yes. The reasoning was that JSON is often too technical for non-technical users. But CSV support could have limitations (for data that makes sense to be include in CSV). For content types with binary attachments, CSV support should uploading those binary files with CSV (e.g. in the same ZIP).
Yes. Making a release after each usable feature would reduce risks. Leaving packaging releasing at the end of the project would be risky. Without a working release, there is risk of the work being wasted. Making a release even with partial features of the original goals would allow use of the release by community and finishing the incomplete features later.
Automated tests would preferred. Those test would probably mostly be integration and functional / acceptance tests.