[AlphaCommerceHub] Does the `PayerDetails`.`CustomerName` field of a response to the PayPal's `PaymentStatus` API request always contain only the last name of the customer?

Yes: a PayPal’s PaymentStatus API request, and a response to it.
So I treat the CustomerName field as a customer’s last name, and CustomerFirstName as the customer’s first name:
the backend «Payment Information» block for a PayPal payment.