How can I access XMLSitemap links from external PHP script?
You'll need to bootstrap the Drupal environment in your PHP script to do this. There are many ways to do this, probably the easiest is Drush's php-script command:

drush php-script my_custom_script.php


This will let you execute any arbitrary PHP script in a bootstrapped Drupal environment.

Once you have your script running your Drupal environment, you can utilize the XMLSitemap API functions for all your CRUD operations.