SQL to order by CPT results by user->display_name w/only user->ID known

If I’m understanding your question correctly, I think you just want to sort your custom column by author, since by default this will sort by the ID. Untested, but can’t see why it wouldn’t work. The display value of the column shouldn’t matter.

add_action( 'pre_get_posts', 'wpse_orderby_author_column' );
function wpse_orderby_author_column( $query ) {
    if( ! is_admin() )
        return;

    $orderby = $query->get('orderby');

    // Change my_custom_column to the name of your column
    if( 'my_custom_column' == $orderby ) {
        $query->set('orderby','author');
    }
}