I believe the issue with:
Sorry, NextGEN Gallery SimpleViewer works only in Combination with NextGEN Gallery
has to do with the class name in the NextGen gallery, the nggSimpleViewer plugin has this line in the beginning that looks for the gallery class, it states:
if (!class_exists('nggallery') ) {
to display that error message. Doing a print_r(get_declared_classes());
shows the class name is actually nggGallery, so I changed the above line in nggSimpleViewer.php to:
if (!class_exists('nggGallery') ) {