The likely cause for why a rehired user is no longer able to access records they worked on before leaving the company is that the records were manually shared with the user. Manual sharing allows granting access to individual records to specific users or groups. However, manual sharing is removed when a record owner changes or when a user's role changes. When a user is deactivated, their role is removed and any manual sharing involving that user is deleted. When a user is reactivated, their role is restored but manual sharing is not. Therefore, the rehired user will not have access to records that were manually shared with them before deactivation. The stage of Opportunity records, permission sets, or record type of Opportunity records are not likely causes for why a rehired user is no longer able to access records they worked on before leaving the company. Reference: https://help.salesforce.com/s/articleView?id=sf.sharing_manual.htm&type=5

When you delete a custom field, the data in that field is stored for 20 days before it is permanently deleted. During this time, you can restore the field and its data from the Recycle Bin or use Data Loader to export the data. Reference: https://help.salesforce.com/s/articleView?id=sf.customize_del_field.htm&type=5

Process Builder is a tool that can be used to implement this request. Process Builder can create record-triggered flows that execute actions when certain conditions are met. In this case, Process Builder can create a flow that executes when a contact with a title of CEO is created and updates the contact's account record with the CEO's name. Reference: https://help.salesforce.com/s/articleView?id=sf.process_overview.htm&type=5

Freezing a user is a way to temporarily prevent them from logging in to Salesforce without deactivating their user record. This is useful when you need to perform some cleanup tasks before deactivating a user, such as removing them from being referenced in a hierarchy field like Direct Manager. Reference: https://help.salesforce.com/s/articleView?id=sf.users_freeze.htm&type=5

To protect data with backups by using Data Export Service, two considerations that the administrator should remember when scheduling export are:
Data Backups are limited to weekly or monthly intervals depending on edition and license type Metadata backups must be run via a separate process such as Metadata API or change sets because Data Export Service only exports data (records) Metadata backups are not limited by sandbox refresh intervals. Data Export Service should be run from production orgs unless testing purposes require otherwise. Reference: https://help.salesforce.com/s/articleView?id=sf.data_export.htm&type=5