Sort wp_query of two post types using meta datefield for one and date for the other – possible?

Try this WP_Query arguments:

$args = array(
    'post__in'  => $all_ids,
    'meta_query' => array(
        'relation' => 'OR',
        array(
            'key'     => '_event_date',
            'compare' => 'EXISTS',
        ),
        array(
            'key'     => '_event_date',
            'compare' => 'NOT EXISTS',
            'value'   => 'dummy_value'  // This is just to ensure this condition gets evaluated.
        )
    ),
    'orderby'   => array( 'meta_value_num' => 'DESC', 'date' => 'DESC' ),
    'order'     => 'DESC',
);