Leo, any article content that is not explicitly downloaded by the user can be purged from the application. Was this content explicitly downloaded, and then at some point it was all purged from the device? Or is this only an issue when the user is offline?
There was a bug fix in March that addressed accessing downloaded content while offline. It was very to cases where you use the restricted collections functionality and try to view any content while offline. You should update the app to see if that addresses the problem.
2) Individual articles are cached to the device as user's read them. The app keeps a rolling cache that can grow quite big depending on the size of the articles, but periodically the cache will purge and rebuild. As mentioned above the cache can be manually cleared using the app's storage settings.