Ok, I have found a workaround for the issue:
Design the book as normal and include the bleed area across the spine for all images/objects (like in the 2nd example above). Before exporting the print file open the Document Settings and untick Facing Pages. Left/right master page properties remain intact. The only issue were images that are going across both pages of a spread – they only appear on one of the two pages after the change and you will have to copy them and paste onto the other page. I hope that makes sense.
Nevertheless, I think the function to 'allow pages to shuffle' and to seperate a spread for loose-leaf printing should be addressed in a future update of Publisher.