DLU is very cryptic. It’s unknown to a lot of people except the developers of the server what is going on inside it. But Darwin (The main person developing DLU) uploaded a video on the progress that they were at.
LUNI wasn’t abandoned, it’s still being developed privately by many people. It was just proved to be too hectic or too much of a hassle to develop publicly.
If you want to help, learn the basics of C++, take a look at the documentaion, get into contact with a LUNI/DLU developer, and don’t be afraid to ask questions. There is nothing wrong with asking questions, DLU developer Xiphoseer got onto the DLU team by asking the original LUNI devs how the program worked. (Tell me if you’d like me to get you into contact with some on Skype.)
Yes, the Clients on this site are the full files (except I hear Nexus 2 is missing some audio files)
Hope that answers everything!