How to add an xml-stylesheet link immediately after the ‘open’ xml tag on the rss2 feed?

There is no hook in the template. See wp-includes/feed-rss2.php:

echo '<?xml version="1.0" encoding="'.get_option('blog_charset').'"?'.'>'; ?>

<rss version="2.0"

You have to replace the template with a complete new one. The template is loaded on the action do_feed_rss2. From wp-includes/default-filters.php:

add_action( 'do_feed_rss2', 'do_feed_rss2', 10, 1 );

Now you can remove the original action and add your own:

remove_action( 'do_feed_rss2', 'do_feed_rss2' );
add_action( 'do_feed_rss2', 'wpse_119139_feed_rss2', 10, 1 );

function wpse_119139_feed_rss2( $for_comments ) 
{
    if ( $for_comments )
        load_template( 'path_to_custom_comment_feed_template' );
    else
        load_template( 'path_to_custom_regular_feed_template' );
}