In older series the showdance used to be the last dance. I can't recall which series it was changed in. I did prefer it when the showdance was last.
They've never done a dance off in the final, just an elimination of the lowest scoring couple on public vote (when there were 3 couples) and no elimination at all when there was just 2 couples.
The judges' scores were for guidance only in the final. The winning couple were the ones who won the public vote. It didn't matter how many or few 10s they got.