I’ve observed the page and I think we can do a workaround.
You can just stick w/ the plugin your site is currently using (Simple Lightbox). Let’s just dequeue the scripts from WPSP’s featherlight so the lightbox is handled by your plugin.
Add this PHP snippet.
remove_action( 'wp_enqueue_scripts', 'wpsp_lightbox_scripts' );
You can place this code on a functions.php of a child theme or using a Code snippets plugin.
With this, Simple Lightbox will take over WPSP’s featherlight as the gallery function.