It's not "cannot be overwritten ever". It's more "you can overwrite it using credentials stored in a safe location and never from the production environment".

Someone will always have permissions to remove or change the entries, just not easily and not in an automated daily process way.

Would it be sufficient that the delete accounts script be managed and merged at restoration time?

The neatest way I heard is to store PII in an encrypted form and then delete encryption keys when a user requests deletion. That requires the keys to be in a different backup pipeline though.

You still have their data and anyone who bypasses the restoration script can get it so no.

This is not true. The prevailing opinion is that backups do not need to be modified and that it’s enough to delete at restore time (and to of course not process the data again).