In case it saves anyone else the time, adding the following filter on pre_get_posts as an admin only code snippet got this working for me.

add_action( 'pre_get_posts', 'my_sort_admin_column' );
function my_sort_admin_column( $query ) {

   /**
    * We only want our code to run in the main WP query
    * AND if an orderby query variable is designated.
    */
   if ( $query->is_main_query() && ( $orderby = $query->get( 'orderby' ) ) ) {

      switch( $orderby ) {

			// If we're ordering by 'shared_counts_total'
           case 'shared_counts':

			// Setting just <code>meta_key</code> is not sufficient, as this 
			// will ignore posts that do not yet, or never will have 
			// a value for the specified key. This meta query will 
			// register the <code>meta_key</code> for ordering, but will not 
			// ignore those posts without a value for this key.
		  $query->set( 'meta_query', array(
				'relation' => 'OR',
				array(
					'key' => 'shared_counts_total', 
					'compare' => 'NOT EXISTS'
				),
				array(
					'key' => 'shared_counts_total', 
					'compare' => 'EXISTS'
				),
			) );

			// Order by the meta value, then by the date if multiple 
			// posts share the same value for the provided meta key.
			// Use <code>meta_value_num</code> since the meta value
                        // for shared_counts are numeric.
			$query->set( 'orderby', 'meta_value_num date' );
				
           break;

		  
      }

   }

}

The nice function using ‘switch’ comes from wpdreamer and was helpful since I had columns from another plugin with the same issue. This answer on stackexchange provided the solution for getting the posts without a shared_count_total to show.

• This reply was modified 2 years, 4 months ago by . Reason: corrections to comments in code
• This reply was modified 2 years, 4 months ago by .
• This reply was modified 2 years, 4 months ago by .

I don’t know why, but the order worked better for me if the ‘NOT EXISTS’ array is before the ‘EXISTS’ array.

• This reply was modified 1 year, 12 months ago by . Reason: typo