@raygj: great post!
FWIW: images, video and audio can be stored in any location in the sdcard. That's because they're managed with
MediaStore, which is as old as android itself, and used for almost every available app out there.
For other kind of documents there's no easy answer as the recommended way of handling them is using the Storage Access Framework, but that's something that almost nobody is using (with a few exceptions, like CC)
Some document readers (most of them?) do ask for permissions on the entire storage and try to scan and parse all documents that they handle. That's the case of Librera, for example. Others rely on a mix of a embedded file manager & import capabilities.
In all cases document viewers are able to read the files, but to be able to write them on a external SDcard you'll need an app that uses SAF, or simply manage the files with an external app or from your computer.