I think that Ellen Tigh could be a cylon, but an older version of maybe Number Six...? Would this fit? Same characteristics... But after all is said and done, I still think that Roslin is the Final Cylon. The mastermind trying to find Earth so they can truly try & genocide all humans. To have her way she took a back seat leadership role as the secretary of education, when Caprica was nuked all other members of government were assassinated (I presume) to put her in the driver seat of president of colonies by default & left a fleet big enough for survival alone to aid her in the search for Earth. I do beleive that it was her that put the thought of finding Earth into Adama's head. I think her cancer is a ploy to be the 'dying leader' to find earth, but when starbuck returned she tried to kill her, but missed on purpose so that she could allow starbuck to search for Earth for her behind the scenes & not lose leadership status as the dying leader...

Sorry for rambling, tired, lots of thoughts spit out at once.... does this make sense though?