That’s quite a transmission range.
I wonder if the human skin jobs of Earth were “human” to themselves. They had kids, as evidenced by the jacks Dualla found.
Then the final five could be “biological cylon descended humans” of the cylons who fled Kobol.
Big leap of how they could be on Earth 2000 years later:
The human beings descended from Kobol cylons are human. They’re mortal. But like many, they don’t want to die. What if these final five created a hybrid type technology to download “human” conciousness into engineered biological bodies? This would be like the cylon mind download, but the “original” mind was from a body born as a baby. Now you have a human stepping into the cylon download immortality scenario.
This could explain why the final five are so much more human. Because, in their FIRST life on Earth, they WERE human.
All you need then is a body that doesn’t age or deteriorate, or a way to replace that body when it fails. The Colonial cylons picked the latter. What if the Earth humans-to-cylons picked the former? Download once to a body that won’t die unless an extreme event happens.
If all the other humans could have that kind of technology, you’d end death - except by murder and suicide. But it’s a kind of immortality.
Just never create the mechanicals who would hate you, and you have the best of both worlds. Human souls, immortality, option to make kids to fill up a new world ...
In the upcoming spinoff series Caprica, which is set some fifty years prior to the events of the current series, the first Cylons are created in more or less the way you've described.
In my view, it's highly likely that the Colonials are themselves "Cylons" (artificial people) created by a predecessor race (the Lords of Kobol), who were in turn "Cylons" created by the original humans from Earth. The Lords of Kobol nuked their creators (the original Humans)... and were in turn nuked by their "Cylons"... who became the Colonials... who were nuked by "Cylons" of their own. "All of this has happened before..."
This time, however, there's a chance that the Wheel of Karma can be escaped. Don't forget that there are still several tropes from the original series left unused. The Ships of Light... Count Iblis... and, significantly, Starbuck-as-Angel.
And then there's Hera and Nicholas to consider...
Anyway, I have a feeling that the final resolution is going to be so far out that nobody will have guessed it. Wouldn't it be wild if poor, dead Billy Keikeya somehow returns and is revealed as the ultimate puppetmaster of fate?