Darth Nihilus: I believe he was just a man, he happened to be on malachor V when the destruction occurred.
The black one? Well, I'm sure whoever decided Nihilus' figure had dreadlocks would agree with you:
However, I daresay that Hasbro has some explaining to do.
Darth Sion: I also thought Lucien Draay was going to become Darth Sion, but I don't think so. In the comics I even saw it alluded to in a most...colorful manner. Several times. However I thought Draay fought it off and swore against it in the comics...in fact I'm almost positive he went to retire on a private estate his family owned for a long time, up on some moon.
In the Great Sith War of 3,996 BBY, Sion served as a Sith Marauder in Exar Kun's Sith Empire. Seeking death, Sion went into battle against the Jedi but continued to survive the encounters. He became convinced he was immortal as he built up a greater and greater tolerance for pain. Eventually, Sion was struck down, but he did not die. Instead, the pain within him welled up to agonizing levels, driving Sion to rise once more and strike down his would-be killer with all his anger and hate. Sion found that he could hold his own decomposing body together with the dark side of the Force. Though this process brought him excruciating pain and forced him to continuously concentrate on his rage, Sion considered it a worthy price for immortality. Kun ultimately lost the war, but Sion survived the fall of the Sith Empire.
In 3,963 BBY, the Sith Acolyte Haazen prophesied that the Jedi Guardian Lucien Draay would teach Haazen's new Sith, suggesting "Darth Sion" as a possible name he could take; Source
Haazen was later defeated and Draay turned away from the path of the Lord of Pain. Source
So there is some Serious Retcon or explaining that needs to be done here.
However I am fairly certain Lucien Draay is NOT Sion's former identity. I agree there is a likeness, however I thought Haazen or Darth Hayze was Nihilus...I was wrong...
I believe the reason Sion looked so young is because Dark Healing has its way of healing by stopping age and piecing you back together well enough so you can continue to function. The power would have to be preventing erosion and decay as much as possible if the body is to remain intact enough to continue to use.
Sion got himself struck down, but I theorize had learned enough by that point about darkside healing and how to preserve and keep his mind intact and continue to control and use his body that upon the blow that would be death, he rose again.