I would recommend looking into
hook_node_validate(). This is a function hook provided by Drupal core that allows you to add custom validation into the form. While you cannot select "required" from the field UI, perhaps you can validate it here and then return an error if the form doesn't provide the correct information for that specific field.
Other things to investigate:
hook_validate - where you can add custom validation functions into the Drupal form array itself.