As far as the final five, remember the Temple of the Five on the algae planet?
The time they died on earth was probably not the first time they have died and been reincarnated. As for the Cylons on earth you could see in the flashbacks that there were many many different people there instead of limited number of "models"
Its my theory after watching this that the "13th colony" left on the run from their creations, humanity, who rebelled and drove them away.
The Cylons then either destroyed themselves OR were eventually tracked down and destroyed by their creations, who conveniently rewrote history (as the victors usually do) and humanity forgot their TRUE origions, beleiving themselves to be from Kobol. (Did anyone ever do testing on the corpses they found on Kobol? I think they just ASSUMED the corpses were human...)
OR... the tested revealed the 13th colony to be Cylon because of specific markers they were testing for to indicate Cylon were present.
Perhaps they were Cylon/Human hybrids from ages past who united after a brutal conflict and mated together allowing a diverse species.
Maybe the Centurions found arround the cylon corpses were not Cylons but in fact "toaster" early model humans or their minions who were deployed to earth to eliminate any surviving Cylons after they nuked the planet?
Possiblities, so many possibilities....
Maybe some of those 2000 year old Centurions are in hiding on earth and duplicated Starbuck, though still does not explain HOW the heck her Viper made it from the storm on the gas planet to crash on earth. I think there is some series Divine intervention going on there.
Well enough of my ramblings...