The Delete permission is missing on the user for Accounts is what is preventing the user from completing the merge. Merging accounts is a way to combine up to three duplicate accounts into one master record and update the related records accordingly. Merging accounts requires the Delete permission on accounts because merging accounts involves deleting the duplicate records and keeping only the master record.

Displaying the related contacts related list and adding the direct field will allow users to distinguish between contacts and related contacts. The related contacts related list shows all contacts that are related to an account, either directly or indirectly. The direct field indicates whether a contact is directly associated with an account or not. By adding this field to the related list, users can easily see which contacts are direct and which are not.
References: https://help.salesforce.com/s/articleView?id=sf.contacts_multiple_accounts.htm&type=5
https://help.salesforce.com/s/articleView?id=sf.contacts_multiple_accounts_direct_field.htm&type=5

Data Categories are a way to organize and classify knowledge articles into a hierarchical structure based on topics or criteria. Data Categories can help service reps quickly narrow down the number of articles by filtering or browsing by category. Data Categories can also be used to control the visibility and access of articles for different users or roles.

The Setup Audit Trail is a tool that allows you to view and download a log of changes made in your org's Setup area. The log shows up to 20 fields of information for each change, such as who made it, when it was made, what type of change it was, and what values were changed. You can use the Setup Audit Trail to track configuration changes directly in production and identify any unauthorized or problematic changes. One of the changes that the Setup Audit Trail tracks is when a user grants login access to another user. This means that you can use the Setup Audit Trail to see if any other administrators or delegated administrators are using the Log In As a User feature. References: https://help.salesforce.com/s/articleView?id=sf.monitorsetup.
htm&type=5 https://help.salesforce.com/s/articleView?id=sf.remoteaccess_granting_login_access.htm&type=5

Query and export the Account History object using Data Loader is how the administrator can acquire the field history data for analysis. Data Loader is a tool that allows administrators to import, export, insert, update, delete, or upsert records in Salesforce using CSV files. Data Loader can handle large data sets and complex data transformations. Data Loader can be used to query and export the Account History object, which stores the field history data for accounts, using SOQL statements.