How do I set the default admin sort order for a custom post type to a custom column?

Solution from a cross post over at StackExchange from @birgire:

The problem is that you run the clientarea_default_order callback
too late.

To fix that you only have to change the priority from the default one
that’s 10:

add_action( 'pre_get_posts','clientarea_default_order');

to the priority of 9:

add_action( 'pre_get_posts','clientarea_default_order', 9 );

But you don’t actually need two pre_get_posts callbacks.

You can combine them:

Example #1

is_admin() && add_action( 'pre_get_posts', 'extranet_orderby' );    

function extranet_orderby( $query ) 
{   
    // Nothing to do:  
    if( ! $query->is_main_query() || 'clientarea' != $query->get( 'post_type' )  )
        return;

    //-------------------------------------------  
    // Modify the 'orderby' and 'meta_key' parts
    //-------------------------------------------  
    $orderby = $query->get( 'orderby');      

    switch ( $orderby ) 
    {
        case 'extranet_sort_office':
            $query->set( 'meta_key', 'extranet_case_office' );
            $query->set( 'orderby',  'meta_value_num' );
            break;
        case 'extranet_sort_date':
            $query->set( 'meta_key', 'extranet_appointment_date' );  
            $query->set( 'orderby',  'meta_value' );
            break;
        case '':  // <-- The default empty case
            $query->set( 'meta_key', 'extranet_appointment_date' );  
            $query->set( 'orderby',  'meta_value' );
            break;
        case 'extranet_sort_type':
            $query->set( 'meta_key', 'extranet_appointment_type' );
            $query->set( 'orderby',  'meta_value' );
            break;
        case 'extranet_sort_IP':
            $query->set( 'meta_key', 'extranet_insolvency_practioner' );
            $query->set( 'orderby', 'meta_value_num' );
            break;
        default:
            break;
    }
}

where we added a main query check and an empty switch case.

Example #2

Here’s another approach, without the switch part (PHP 5.4+):

is_admin() && add_action( 'pre_get_posts', 'extranet_orderby' );

function extranet_orderby( $query )
{
    // Nothing to do
    if( ! $query->is_main_query() || 'clientarea' != $query->get( 'post_type' )  )
        return;

    //-------------------------------------------  
    // Modify the 'orderby' and 'meta_key' parts
    //-------------------------------------------  
    $orderby = strtolower( $query->get( 'orderby') );  
    $mods = [
        'office' => [ 'meta_key' => 'extranet_sort_office',           'orderby' => 'meta_value_num' ],
        'date'   => [ 'meta_key' => 'extranet_appointment_date',      'orderby' => 'meta_value'     ],
        ''       => [ 'meta_key' => 'extranet_appointment_date',      'orderby' => 'meta_value'     ],
        'type'   => [ 'meta_key' => 'extranet_sort_type',             'orderby' => 'meta_value_num' ],
        'ip'     => [ 'meta_key' => 'extranet_insolvency_practioner', 'orderby' => 'meta_value_num' ],
    ];
    $key = 'extranet_sort_' . $orderby;
    if( isset( $mods[$key] ) )
    {
        $query->set( 'meta_key', $mods[$key]['meta_key'] );
        $query->set( 'orderby',  $mods[$key]['orderby']  );
    }
}