Does Mend use a purpose-built, hardened operating system (in the case of an appliance) or offer a hardened operating system configuration?

No.
Mend does not use a purpose built hardened operating system. We rely on standard AWS and Azure cloud management and comply with their security standards.
Please see the following for more information: Encrypting Amazon RDS resources - Amazon Relational Database Service
https://docs.microsoft.com/en-us/azure/security/azure-security-management