Never count out complicated and even bizarre solutions on BSG, certainly when said solutions offer dramatic payoff - Ron Moore is running the show, and didn't Grace Parks say that she had to read one of the final script three times and have RDM explain it some more to "get" it? Sounds pretty complicated to me, whatever it was being explained.
Anyway, on this board people usually are willing to consider the head Cylons, the entire human race, Kara's Viper, Galactica itself and even body-jumping ghost Cylons as the fifth Cylon.
For some reason, all those seem more plausible to many here than some trickery with model numbers and people not being what they seem at first, allthough this has been happening frequently in BSG. At least Boomer could easily have been one of the human shapes seen and recognized by D'Anna in her vision, and D'Anna would have had very good reason to be surprised.