Poland also cooperated with Nazis before 1941. So did much of the rest of Europe and Britain. And no, it wouldn't have slowed Germans down, just like it did not slow them down in 1939. In general, European countries did not put up much of a fight, and some (Romania and Finland, for example) explicitly and willingly fought on Hitler's side. As to the crimes, name me an Axis or Allied country that did not commit horrific war crimes in that war. I'll wait.