Actually, Nihilus did use Sever Force on Traya when he and Sion rebelled. Why he did not use it on Sion, I do not know. Perhaps whatever humanity still remained within him felt no need to do so. This could explain why he didn't outright kill Traya. Or perhaps he was using Sion to draw out the Jedi. I don't know why Nihilus would let Sion leave.
And you probably wont get the mask. Very, very, very rare piece.
When did Sion confront Nihilus, I thought that was cut content? (which he died in anyway, or at least was severely battered)