Why would running an admin check throw a 500 error? I’m calling is_super_admin();

From your pastebin:

if (!function_exists('wp_get_current_user')) {
        include_once( ABSPATH . 'wp-admin/includes/pluggable.php' );
}

if ( is_super_admin() ) {      
        add_action( 'admin_init', 'myplugin_register');
        add_action( 'admin_menu', 'myplugin_menu' );   
}

There is no file at wp-admin/includes/pluggable.php. pluggable.php is at wp-includes/pluggable.php, but I see no reason you should have to intentionally include that file. It load front and back end.