The human remains found on earth are actually cylons who fought against robotic centurions (earth variant) and perished in a nuclear holocaust 2000 years before BSG takes place. The Final Five are actually the “Earth Cylons” who lived in the planet before dying and reincarnating 2000 years later in the Colonies.
Kara finds her crashed Viper and her corpse somewhere on Earth but she is not the Final Cylon. Ellen Tigh is the Fifth and Final Cylon but she died back in New Caprica.
Duella commits suicide after loosing all hope upon finding Earth. Roslin loses her faith while the rest of the fleet also lost hope.