... this would probably mean that the final 5, if they've lasted this long, are probably 1) from earth and 2) are the product of a human-Cylon friendship concluded at the end of the previous cycle.* This would mean they will help the humans not hurt them. It also explains the whole Starbuck going to earth plotline (I don't think she's in Lee's head, and if she is, bummer; instead she must have been scooped out of her ship before it crashed by the same cylon transport that Lee saw). It also supports your notion of the last hidden Cylon being somebody like Adama, who will bring humans and Cylons together in the current cycle (interestingly, earth was his idea). Probably not Roslyn though--like Moses she appears to be fated to never see the promised land.

*but if true how could the current cylons know about these older cylons, if from different cycles, have never met face to face?