“we were allied with the Soviets after all, the only state to kill more Jews than the Nazis”: Can give from where you pick this information? I doubt the Soviets killed around 5 millions Jews in around 3 years in camps specially designed for that.
Can you provide the Soviet equivalent of the Bodensee conference? It will help.

About the “ethnic cleansing” of the German Minorities:
First, you can’t compare the expulsion and the genocide. I know that today it is a word used and abused but…
Second, just study how many volunteers these “minorities” gave to the Nazi, and what attitude did they adapt during the German Occupation. That will gave you a little clues on the reason why the expulsion.
In term of politic, the reason evoked by Hitler for the invasion (then annexing) of Czechoslovakia was the protection of the German minorities. I think he took the same reason for Poland, but this will have to be confirmed.
So, after the WW2, well, it was decided by the victims of the Nazi aggression that this won’t be an pretext again.