If she was the empress of the Holy Roman Empire, she can lead a civ with this name, HRE.
As we can see in the map below, the "Germany" take a part of Austria , but not all of that, and more, Austria was a independent kingdom who is just partially in Germany and the other half control other european nations as Hungary.
Also, there was wars between Prussia and Austria. If Germany be a thing before 1870, as you said, theses wars should be civil wars, but it isn't, it was formal war between nations.
So, I think put Maria Theresa as leader of Germany is extrapol too much the concept of Germany... Why we don't extrapol the nation of concept of other nations? As using Guarani leaders as Sepé Tiaraju as leader of Paraguay? Or Brazil have it's unique unit before the independence as using Bandeirantes....
As I said in other thread, we can even use Teodoric, king of Visigoths, to be leader of German if we extrapol too much the concept of Germany.
But, as a historian nerd, I think we should pick Bismarck ever as leader of Germany, because he is the only one who isn't controrversial.