Unit testing - no unit testing yet but you can test out mock request data by using the Run Options tab in the Napkin editor.
Organization - folders and workspaces coming soon, but if you have any other ideas for this we'd love to hear it!
Right now you are restricted to single "file" per endpoint. In the future we plan to enable Napkin functions to reference each other which we hope will mitigate some of this limitation.
Local development is not currently an option; however, we plan to add Github integration some time this year, which would enable this. How we prioritize it will depend on how many users ask for it.
Thanks so much for the questions!