He's too obivious, Baltar see's Six as he has for the most part lost the rational part of his mind and whenever he's faced with a decision or a choice he looks to his memory of her, which is manifested as a vision, to help make his choice. As you have seen he is now seeing himself because he is being worshiped as a semi-god and he is locking on that to make his decisions.
As for the 12th Cylon well I thoight Starbuck at one point but that also is too obivious.
The Prez is a good choice but yet she is supposedly dying so there is a question there, does she know and has she been lying the whole time, I think not.
My 2nd guess would be the Doctor as he is old enough to be the first Cylon human looking model but he has not played such a major role as to have the shock of discovery that the fans like me expect.
For now I am unsure who it is so time will tell

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