Sure. You might need to create a custom ruleset for the address fields, and you can run it manually. Info about the rule configuration: docs.civicrm.org/user/en/latest/common-workflows/… Manual execution: docs.civicrm.org/user/en/latest/common-workflows/…

What is wrong with the API solution that i mentioned in my answer? In the API explorer tool, you can try out everything, you can execute the API calls and check the response. It is possible to select one particular row from the database with API. Also possible to update one particular row with the API, so it could fulfill your need. And it is documented well. docs.civicrm.org/dev/en/latest/api

What are your CRM and CMS versions? I could not reproduce this issue on Civi 5.37.1 & Drupal 8. I have created an event form with required fields. Then submitted the form without filling it. The client side validation printed the 'This field is required.' error messages for me only once / required field.

The custom search with excluding all the groups supposed to be fine. It lists me only those contacts that are not in groups.

The template content is dropped with the reset only those changes are expected that were added to the Newsletter Template. But the recipient, subject, etc setup is kept. I think it is an acceptable loss if you want to make sure that the important changes are stored in the main template and the current mailing is originated from the latest version of the template.