Explanation
To import data into a non-sendable data extension without a primary key, the admin could use the Overwrite import type3. This import type will delete all existing records in the data extension and replace them with the new records from the import file. This import type does not require a primary key because it does not need to match or update any existing records. The other options are incorrect because:
Add Only. This import type will add new records to the data extension without affecting existing records. However, this import type requires a primary key to prevent duplicate records from being added3.
Add and Update. This import type will add new records and update existing records in the data extension based on the primary key. However, this import type requires a primary key to match and update the existing records3.
Update Only. This import type will update existing records in the data extension based on the primary key. However, this import type requires a primary key to match and update the existing records3.

To configure multiple business units in Marketing Cloud to use the same SFTP directory, the most efficient method is to copy the parent SFTP user credentials into each child business unit. This approach ensures all business units access the same SFTP directory for file transfers, maintaining consistency and simplifying management of file access across the business units. This setup avoids the complexity and maintenance of separate SFTP users for each business unit, streamlining operations.
Reference: Salesforce Help - SFTP Setup with Business Units

To resolve the issue of "Insufficient User Permissions" while setting up Marketing Cloud Connect, the action that should correct this issue is to apply the appropriate user mappings in the CRM configuration. This involves ensuring that the Salesforce user is correctly linked with the Marketing Cloud user credentials and roles. The error indicates a mismatch or an incomplete configuration in linking Salesforce and Marketing Cloud users, which user mappings can correctly align.

Explanation
Linking the Customers data extension to the data model using Customer ID and linking the Orders data extension to the Customers data extension using a One-to-Many relationship are the best ways to configure Data Designer to allow this data to be used within a Journey. Customer ID is a unique identifier that can be used to link data extensions in Data Designer. A One-to-Many relationship means that one customer can have multiple orders, which reflects the data model scenario.