I want suggest one small improvement. In the case a sidebar is defined in a post/page but itself is set to status draft the method $MultipleSidebars->is_active_sidebar() returns true although no sidebar is shown. In this case it would be desirable to have a possibility to display a fallback sidebar.
This could be reached if method $MultipleSidebars->dynamic_sidebar() returns a boolean value from the calls to the original WP function dynamic_sidebar(). As this call may executed multiple times in your method I use following modification: