I have decided to go with minimal changes and see what happens. What this means:
On a device manufactured by Amazon:
- The default book folder is "Books"
- Epub is no longer enabled by default, but nothing will change if it is already enabled.
- Launching a book in the native reader will open a dialog asking if it is OK to reset the current position to the beginning. There is a check box not to see the dialog again.
On a device not manufactured by Amazon:
- The default book folder is "Calibre_Companion"
- Epub is by default, and is the top-most choice.
- Launching a book in Kindle for Android acts as today; K4A is opened but no book is opened. There is no dialog and the reading position is not reset.
In either case, if the book will be opened in a non-Amazon app then CC behaves as today.
These changes allow CC to open the native reading app on Amazon devices for all amazon extensions using the same behavior currently used for .mobi files. The dialog box should help avoid surprises. The behavior for other book formats is not changed, so people who are using other reader apps should see no changes.
Given that we haven't had any comment about how .mobi and CC behave on Amazon devices (the position is reset), I don't think that these changes will create controversy. We will see.