1 & 6. I use widgets with Mantano and Play Books for the simple fact that they are quick access shortcut to resume reading where I left off. Granted it does not take much more to access the app, to return to reading.
2, 3, 4 & 5. I have seen/used 1x1 widgets where upon adding the widget to the home screen the widget takes the user to the app to select which book to display. The 1x1 widgets are small, but usable and cannot typically be resized. The widget would have to be Removed/Added to select a new book. Adding multiple widgets allows the use to select several books to have quick access to.
Play Books also provides a 2x3 widget that displays several books stacked, that can be scrolled through with an upward/downward gesture. The widget is resizable. I find this widget nice... however, I do not generally read multiple books at a time. Perhaps if a reader selects a CC or Custom Group to display books from if a similar stackable/scrollable widget is used?
Maybe a button on the widget to manually refresh the displayed books like the ComicRack for Android app uses?
....
Issue that comes to mind: A CC widget would be unnecessary for users who only use Play Books unless Play Books is able to be selected as default reader for epub/pdf through CC, which I do not know that it can. Even then if it can, the user must upload the book to Play Books first, and in that case would be better off using the Play Books widgets directly.
I've not experience with other reader apps to comment.
Here are a few examples of widgets in use.
Edit: On a side note - The option to "Clear default reader apps" may not be working. I had selected Mantano as my default reader for epub but CC still remembers and does not allow to changing of the reader app.