Poland's mistakes included spreading out their forces in an attempt to defend the whole border. What if they instead choose to concentrate their forces at key choke points and attempts to bleed the Germans white. In this TL, they would also disband their 12 cavalry brigades and convert them to infantry. Lastly, if the Germans are bogged down and are obviously struggling to beat the Poles, what are the odds of Britain and France attacking through the French Border? I suspect Stalin would be hesitant to invade if he was uncertain of Anglo- French intentions.