Just in case this could be useful during further improvements:
https://learn.microsoft.com/en-us/windows/win32/secauthn/sspi-kerberos-inter... suggests that continuous buffer encrypted by gss_wrap() could be decrypted by passing buffer with type SECBUFFER_STREAM to DecryptMessage(). Since neither dlls/msv1_0 (NTLM) nor dlls/kerberos support SECBUFFER_STREAM current approach with detecting security package and manually constructing data/token buffers is a working solution.