from my experience kindly find below some info:
1-your answer is good, NDLP is used for non windows OS because the HDLP work only on windows OS
2-NDLP Discover will search in the entire network and not only where the agent is installed
3-NDLP manager can control HDLP also
4-with NDLP Monitor you can monitor all trafiic going through SMTP,HTTP,FTP,POP3...protocols so you can fine tune your HDLP policies and you can monitor what the non managed PCs are sending using these protocols.
5-NDLP prevent is integrated with Mail Server and proxy to stop leakage from non managed PCs.
I hope this may help.