At the very least, any scenario that could somehow save Poland would require:
1) Britain, France, and Poland begin immediate war planning after the Polish guarantee is made. Both sides know exactly what is required of them in the event of a German attack. Poland is assurred that its territorial integrity is confirmed and that it is not necessary to fight the Germans everywhere since a lack of fighting in the corridor won't cause the Allies to not support Polish claims there. The idea is for Poland to hold out for six months to give Britain and France enough time to build forces and invade Germany to compel an immediate end of the war.
2) France commits, as part of war planning, to invade Germany no later than 24 or 48 hours after German troops cross the border into Poland. Units to carry out the invasion are designated and will be prepared to do so.
3) Polish strategy is not to fight the Germans at the border, but to deploy the bulk of their forces behind natural defenses like the Vistula River. Polish units west of that line will fight a delaying action, but the intention is that Vistula will be the main line of resistance where the Germans can be stopped, and as alast resort the Romanian Bridgehead.
4) The Polish government emphasizes the danger that German aggression holds for Poland, and properly instructs the population on what to do if attack comes. Above all, they need to make sure that refugees don't clog the roads so that the army can move.
5) In order to give the Poles adequate air defense, and with the belief that Poland can hold out, Britain and France transfer some fight squadrons to Poland with their British and French crews, in order to make sure the Polish Air Force can hold out and do significant damage to the Luftwaffe.
6) Britain, being unable to raise sufficient land forces for a ground invasion, prepares for an aerial attack against military targets in the west.
7) When the war begins, the Allied offensive goes as planned. While the Germans make strong headway into Poland, French forces move into Saar. As the Germans beging encountering strong resistance near the Vistula, French commanders realize that the Westwall is a hollow shell and that France can advance to the Rhine and beyond.
8) As the Allied offensive develops, the German generals panic and begin stripping the German units in Poland in order to build a true defense in Germany. German offensive in Poland stalls.
9) Stalin, seeing that the Allies are putting up significant resistance, decides it is too risky to activate the secret protocals of the M-R Pact.
10) As the French offensive continues to move, the German generals realize the war is hopeless. They send secret messages to the Allies that they will overthrow Hitler in exchange for the borders pre-Munich. The Allies agree.
11) German generals execute a coup-de-tat and kill Hitler and other senior Nazi leaders.
12) Peace is made, and Poland comes out of it intact.
The above is perhaps barely plausible, but it requires the leaders of Britain, France, and Poland to be a much higher caliber than they were and to understand the situation a lot better than they actually did. Even if the above outlines were followed, it's possible that any number of steps needed to get the final result simply fail.
So while I think a defense of Poland is feasible, I don't think it very realistic. The leaders of Britain and France in particular were unwilling to recognize the true danger they were in and that strong collective defense was their only option.