action hook wp_head higher priority as all other plugins/hooks

I just skimmed through the WPSEO_Frontend class and found the wpseo_robots filter for the meta robots content string.

You might try something like:

add_filter( 'wpseo_robots', function( $content )
{
        // your logic here ...
        return $content;    
} );

to override the robots meta content from that plugin.

ps: You should consider moving the add_action out of the class constructor.