Summary of my opinon on Satele: Pardon my French, but she's a bi.tch! And part of the reason I stopped believe Jedi= good and Sith= bad. Nope, for me Jedi= 1 philosophy, Sith = another philosophy.
Pairing Theron Shan: Well, Theron ends up with Crimsen because they're a like in the sense they come from Force using families and end up going a different way. Also, they're both spies so they are similar, and can complement each other given they're spies, but trained differently. Then of course is the fact that Crim is as lightside as a Sith pureblood can be with some dark tendencies. The two connect.
Why a Sith?: I think the answer is simple from Theron's end- A Sith represents 'forbidden love affair' to someone who's a descendant of a Jedi. Why not?