Can we treat MT707 received through MT798 as an authenticated message from issuing bank?

No, an MT707 received through MT798 cannot be considered an authenticated message from the issuing bank. The MT798 is a free-format message that does not carry the same level of authentication and security as standard SWIFT messages. For an MT707 to be considered authentic, it should be received directly from the issuing bank via the appropriate SWIFT channels, ensuring its integrity and authenticity.