Good points - one thing I emphasize is that France and Britain didn’t merely fail Poland, but in 1939 it would have seemed to almost anyone (including many German officers) that the ‘guarantee’ was worth a lot. It was Hitler’s uncanny sense of the mental and moral weakness of his foes that led him to expect that there would not be serious action in the west while he invade Poland. However, what the Poles expected because they had been PROMISED it by France and Britain was an aggressive land and air attack on German forces in the west. France was supposed to have up to 70 divisions available on the western front compared to only 25-30 understrength, 2nd-tier divisions for Germany. Britain was known not to have much of an army but could have contributed a few divisions in a renewed “British Expeditionary Force” — the primary contribution from Britain was supposed to be their air force. That is what the Poles were expecting, that Germany would immediately be forced into a 2-front war with an inadequately defended western frontier. In reality, as we know, the Poles were badly betrayed as the French command decided it was better to stay hunkered down (they advanced only 5 km or so into Germany, with little resistance, and then stopped). The French saved on casualties in the short term, but of course they allowed Hitler to dispose of his adversaries one by one instead of facing the 2-front war right away.
What you said.