Frederick the Great is shown playing a flute concerto with Carl Philip Emmanuel Bach in Dresden in September 1756. Frederick was King of Prussia; Dresden was the capital of Saxony, a different country. In September 1756 Frederick was at war with Saxony.
After Frederick's flute performance is interrupted, he turns back to C.P.E. Bach and says, "Maestro Bach, Allegro." Allegro means fast. They're actually playing Andante, or moderately slow.
Frederick the Great decides to seek alliance with Britain against Russia in June 1757, well after conquering Saxony. In reality, Frederick concluded a pact with the UK in January 1756, precisely to upset Russia's efforts to form an alliance. Without British backing, Frederick could never have attacked neutral Saxony in the first place.
Catherine is shown rescuing a young lieutenant Grigori Orlov from prison in 1757 before he goes off to war. In fact, Catherine met Orlov when, as Peter Shuvalov's adjutant, he escorted a high-ranking Prussian prisoner of war from the front to St. Petersburg. Also, far from landing him in jail, Orlov's affair with Shuvalov's mistress greatly enhanced his reputation in the capital, though understandably it did cost him his job.