Inserting post_id into guid before wp_insert_post

Question

Why would you need that? ID in a database is auto increment – it manages it’s value itself. If your target is to set custom guid, you should update the inserted post as soon as it is created via wp_update_post right after your $post_id = wp_inserted_post($mypost); like this:

...Your code above...
$postid = wp_insert_post($mypost);
$data = array(
   'ID' => $postid,
   'guid' => get_option('siteurl') .'/?post_type=ai1ec_event&p='.$postid.'&instance_id='
);
wp_update_post( $data );