Obviously five minutes of internet research isn't definitive, but so far as I can tell there is exactly no evidence for the claimed fact that "German citizens... were being ethnically cleansed (and massacred) in Danzig". It seems that 75 years on people are still falling for Nazi propaganda. This is bizarre, because Hitler himself clearly stated that Danzig was not the issue, but Lebensraum.
As to not declaring war on the Soviet Union; there was a treaty between Britain, France and Poland guaranteeing that the first two would declare war if Poland was invaded by Germany. It was perhaps an aberration that they did not also declare war on Russia, but then frankly for the first six months or so of the War Britain and France did little other than meek posturing anyway. And it was perhaps a good thing that we did not declare war on Russia also: it would have meant nothing in practice for the first few years, and then, when later in the war, entirely predictably, Germany turned on Russia, their past allies, and we gained a new ally in the fight against the aggressors.
Any idea that somehow this is all Poland's fault is utterly ludicrous.