Ok... Sadako's reign began in 1870. We learn that Clandestine has been a concience without a person for 135 years, in March of 2005. The math puts Clandestine's loss at almost the precise time that Sadako took over the Dream Realm... seems obvious that the math adds up, so I'd consider that pretty much a done deal (unless the mindfuck goes way deeper than we imagine).
As for why Sadako did what she did... it seems obvious that she needs a Real Worlder who's severed their connection, for some nefarious plan. She lured Lia into the Dream Realm and tried to keep her there on her own. When Lia's attention was drawn back to the real world because of George, Sadako used an illusion of him to get Lia to seal her doorway home. When Lia proved impossible to turn, for whatever reason, she used Lia to seduce Fiona... who now seems eager and ready to fight Sadako.
If you ask me, that's EXACTLY what Sadako wants. I think realworlders that voluntarily enter the Dream Realm have immense power, but that said power can't affect another realworlder. Maybe it's something like Highlander... kill another realworlder, take their potential for power. Sadako wanted Lia to fight, but when she refused, she used Lia to get Fiona so drawn into her first coma-dream, she would stop at nothing to get back.
What I want to know is... how did Sadako know that Fiona would find the book, and with it, a way back in? Does she have some way to extend her power outside of the Dream World? Did she (or one of her agents) put Lia's book exactly where Fiona would find it, to bring her back in... with so much hatred towards Sadako, that the duel between them would be inevitable?