Saga of Tanya the Evil: The Movie is 100% canon, which covers the next arc after this arc.
1. Tanya and Mary Sue will have a fight in that arc.
2. No.
3. During the fight, it revealed that Being X gave Mary Sue 3 blessings to troll Tanya, which made Mary Sue gain equal footing to Tanya and gave Tanya a hard time. The top brass after hearing the report about Mary Sue (minus the blessings), realized there was an Ace in their rank, plus her naiteve (stupidity) and simple-minded: to only kill Tanya, which was easy to order around. And they used her pretty much a lot until the end of the war, which caused resentment among her comrades and fellow soilders because she was the type that willing to let 100 Allied soilders die just to get Tanya.
4. Nope. Like I said earlier, all sides of the war were suffered massive loss, the Empire had no legs to stand and neither did the enemy countries, Tanya made sure of that.
5. Tanya and Zettou (the monocle general) knew fully well the Empire was over when they got atacked on all fronts; the not-US entered the war with Aircraft Carriers, they agreed to end the war as soon as possible, but Rudersdolf and most of the Empire wanted to continue the war due to Tanya and her Elite Army' successes. As a result, Zettour ordered Tanya to assasinate Rudersdolf, in order to unite and lead the Empire in one last war per Tanya proposed which helped the Empire manage to sue for peace. Zettou burned all documents related to Tanya, painted himself as a mad man and a "war criminal" by taking the responsibities for the Empire's "war crimes", including Tanya's, giving her and most the Empire's officers a chance to survive. Burnt documents+ the skepticism of public about Tanya and propagandas by all sides: I mean, the Devil of Rhine, one who inflicted terror in the battlefield, was a girl!? 9-10 years old!? She was only a soidler? That's absurd, how could a single soilder be able to do anything? It was a lie, blah blah...Thus, Tanya was scubbed from history, she safely moved to the US under the fake name Tasha
6. She was not under scrunity because she was right. Because the top brass of Empire did not order to annihilate the Republic Field Army, they managed to retreat back to their Africa colonies, which promped them to restart the war, let not-US, not-UK and not-Soviet joined the war against the Empire, thus doomed the Empire.