[python and Dropbox API] New users cannot access files in my /app/ folder

Thanks for the detailed question! The "development"/"production" status wouldn't be the issue here, and most of the rest of your understanding is correct. (And if you don't need to connect to more than 50 users, you don't need production status regardless.)

It sounds like the issue here is that each user receives their own distinct "app folder". When a user authorizes an app with the "app folder" permission to access their account. Dropbox automatically creates a new app folder in the user's account. Dropbox then returns an access token specific to that app-user pair, which will only allow access to that user's own app folder. Each app folder starts empty, and the app folders for different users do not sync with each other.

If you want to start each user out with some content in their app folder, you'd need to have the app upload that file content when they first connect to the app.

Or, if you want all users of your app to connect to the same content and have that sync, you'd instead need to register and use an app with the "full Dropbox" permission and invite all of the users to a single shared folder that you've previously configured for them. Apps with the "full Dropbox" permission don't get app folders, and can instead access anything in the connected accounts, such as shared folders.