Is the show dance usually last
Why was their no dance off for the last 4
How come the winner only once got 40

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.

Used to be show dance last, but not since 2009?
They kept to viewers only vote, hence no dance off and scores not counted/guide only.

This is so we are not deprived of epics like Lisa & Brenda

They had already been eliminated when they did their showdance - the third placed couple still got to do their dance as a kind of final send off.

They must have seen and thought 'there's no way we can pass this up'