I would like to ask if Windows Server 2012 R2 is able to authenticated NTP implementation (acting as a client) and using a defined (not Kerberos or AD related) symmetric key for authenticating and exchanging the NTP packets.
To my knowledge SNTP means "Simple Net Time Protocle" and not "Secure". Also, the Windows NTP service can only use Kerberos as a mean of authentication.
The Simple Network Time Protocol (SNTP) is a simplified time protocol that is intended for servers and clients that do not require the degree of accuracy that NTP provides. SNTP, a more rudimentary version of NTP, is the primary time protocol that is used in Windows 2000. Because the network packet formats of SNTP and NTP are identical, the two protocols are interoperable. The primary difference between the two is that SNTP does not have the error management and complex filtering systems that NTP provides. For more information about the Simple Network Time Protocol, see RFC 1769 in the IETF RFC Database.
No. Microsoft has a custom authentication mechanism in their NTP implementation of the Windows Time Service, but not as you describe. It's also used only for authentication. The Authenticator and ExtendedAuthenticator extensions are published here:
[MS-SNTP]: Network Time Protocol (NTP) Authentication Extensions