Surely the final cylon being boomer is flawed because Boomer has re-incarnated twice I think. If it is actually a different model from the 8s rather than simply a different set of memories/programs then the 7 would realise, because they would have 8 different models in the resurrection ships rather than 7. I think we can be fairly sure that the 7 and the 5 have had nothing to do with each other up until now.
I reckon it will be Zak Adama. The Zak legacy kind of died by the end of the first series, with no more talk of kara being the last link between the surviving Adama's and their dead brother. If not then I don't know, it could still be Kara, or one of the children isn't a bad idea. Why not Ellen? Balthar would know though, because he did the test on Ellen and claimed she was human, but then said that no one would ever find out the truth. If Balthar knows then he's not acting like it.
I still have a problem with the fact that the XO is a cylon. It means that 10 years after the first Cylon war they had already evolved into skin jobs - or that the last 5 skin jobs come from somewhere else (like earth?). And it means that Tai's military record from fighting in the first cylon war must be entirely fabricated - because he was "re-instated" by Adama when he had become a major.
And what about the virus on the earth satellite thing that infected all those cylons? Could that virus be wondering around earth? Perhaps the radiation has killed it.
I don't quite know what they were hoping to find on Earth anyway? A fully working colony, which would then be drawn into the cylon battles themselves and likely be wiped out along with the colonials? An empty planet, to hide on until the bad cylons caught up with them? I get the impression it is all too radioactive to be a good place to settle - why not just go back to Kobol, that seemed lush enough, with trees and heritage and stuff. Perhaps they need to find the Opera House on earth and discover their destiny - which must have something to do with the cylon/human hybrid babies (of which there is one male and one female, who'd have guessed).
As someone has already pointed out, why is Roslin still alive? It isn't as if she is still cured (why doesn't she just used the miracle babies blood again?), she is actually nearly dead and yet she is standing on earth. I don't get it.