Poles planning to attack to Czechs and the thing about poles were silent allies of Hitler were products of, well - propaganda and Stalin's paranoia. Wouldn't have happened. If the Czechs fight and the French mobilize, the Poles would mobilize as well and act against the Germans. Which would lead to a two front war for Germany which would be lost.
Stalin would have limited options in aiding the Czechs due to the lack of a common border. If he was to try to 'make' a common border, the most likely result is Poland redirecting forces east and fighting. And it would be hard for me to imagine that Germany would not try to use the opportunity to save itself from the fire by begging for a ceasefire and joining the fight with the Soviets along with the western allies.
Czechoslovakia is likely to be able to hold the line in the Sudeten mountains for some time, but would not be able to do so indefinetly. Relief in the form of a second front would have to come, and soon - technically the Czechs and Slovaks would be fighting on not quite a common ground, the Sudeten Germans being NSDAP followers, many of whom were members of selbstschutz. Sabotage and obstruction in fighting as well as war production could potentially cause as much damage to the war effort as the Wehrmacht.