Meta query with multiple custom fields for archives page ordering problem

Question

I'm modifying the query for archives pages so that the posts showing up on year, month or day pages are queried by there custom field (start-date and end-date) and are sorted by custom star-date field. Here's my code :

function display_posts_on_archive_pages( $query ) {
    if( $query->is_main_query() && $query->is_archive && !is_admin() ) {

        $year = $query->query['year'];
        $month = isset($query->query['monthnum']) ? $query->query['monthnum'] : NULL;
        $day = isset($query->query['day']) ? $query->query['day'] : NULL;

        if($query->is_year){
            $value = array(mktime(0, 0, 0, 01, 01, $year), mktime(23, 59, 59, 12, 31, $year));
            $compare = 'BETWEEN';

        } else if($query->is_month){
            $next_month = sprintf("%02d",$month+1);
            $value = array(mktime(0, 0, 0, $month, 01, $year), mktime(0, 0, 0, $next_month, 01, $year));
            $compare = 'BETWEEN';

        } else if($query->is_day){
            $value = mktime(0, 0, 0, $month, $day, $year);
            $compare = '<=';
            $compare2 = '>=';
        }

        $meta_query[] =
            array(
               'key' => 'if_events_startdate',
               'value' => $value,
               'compare' => $compare,
              );

        if($query->is_month){
            $meta_query[] =
                array(
                   'key' => 'if_events_enddate',
                   'value' => $value,
                   'compare' => $compare,
                  );
            $meta_query['relation'] = 'OR';
        }

        if($query->is_day){
            $meta_query[] =
                array(
                   'key' => 'if_events_enddate',
                   'value' => $value,
                   'compare' => $compare2
                  );
            $meta_query['relation'] = 'AND';
        }

        $query->set( 'year','' );
        $query->set( 'monthnum', '' );
        $query->set( 'day','' );
    if(!$query->is_month){
        $query->set( 'meta_key', 'if_events_startdate' );
        $query->set( 'orderby', 'meta_value_num' );
        $query->set( 'order', 'ASC' );
    }
        $query->set( 'posts_per_page', '10' );  
        $query->set( 'meta_query', $meta_query );
    }
}
add_action( 'pre_get_posts', 'display_posts_on_archive_pages' );

For Year and Day pages everything is OK, but for Month pages the sorting won't work (that's why I've put the *if(!$query->is_month){* in the $query->set. At the end for month pafges I have the right posts but without the start-date sorting... And if I delete the *if(!$query->is_month){* statement I get posts even if they are not included in the month queried (ex.: posts of september in august...)

If anyone has a clue... didn't find my solution for the moment. Thanks