I do not see the Polish government making this concession. Judging by their behavior OTL, they would rather go down. They accepted going to war when it was clear that Germany could overrun them. They refused to even consider the alliance with the Soviet Union. The only possibility, and that is a slim one (ASB almost) is that Soviet Union si more aggressive and rattles the saber on Poland. Even then, I think they would refuse the German help in the same way they refused the Soviet help.
And I find it hard to see why would France and Britain abandon them. Would this be prior to Czechoslovakia and Munich?
In OTL the Poles had France and Britain giving them a strong guarantee - they thought that they could resist the German advance for long enough that France and Britain would smash Germany in from the West. Without a guarantee, the choice would be much harder. Certainly, if Germany behaved as OTL in Czechoslovakia prior to putting the squeeze on Poland, I think Poland would probably fight, as they would have evidence of just how little Hitler cared to keep the agreements he made. If Czechoslovakia goes differently, however, then an isolated Poland may well choose not to fight (or rather, not to fight right away - Poland might make concessions to buy time, but I really can't see them meekly accepting German domination, though said fight may not involve bullets).
OTL the Germans did try to bring the Poles into an anti-Soviet alliance. (While also demanding to cut the Poles off from the sea.) The Poles didn't trust the German motives. The only way to change that is for Germany to actually have better motives, I think. So yes, I agree, it's almost ASB for Poland to enter into such an alliance.
Britain and France could very easily abandon Poland.
IOTL, France at several times considered dropping Poland for the Soviets (fell through because the French didn't think they could trust the Soviets). Also, there were some in France (Laval, for example) who thought that they should stop with the whole "great power" charade and accept being Germany's sidekick.
As for Britain, the British foreign office had an almost irrational hatred of Poland. Ever since 1918 they had been thinking of Poland and the rest of Eastern Europe as "Germany's natural area of influence". Naturally, much ink was spilled bemoaning the obstinate Poles who refused to kneel to the superior and civilized Germany. Then Munich happened, and the British began to reconsider their opinions on Germany and Eastern Europe...
So there are a few ways either could decide to stay out of what happens in Poland.
Note, I don't think any of this is terribly likely. I was just outlining what I saw to be the prerequisites for Poland agreeing to concede Danzig.
Finaly, once Germany settles things with Poland one way or the other, I do not see the need to confront France. If the allies have already decided to give Germany a free hand in the East, as is obvious from all this, the entire rationale Germany had for fighting the French is gone.
Hm. If the PoD is a Lavalist France that rolls over and shows its belly when Germany barks, signs up to a war with the Soviet Union, gives up Alsace Lorraine upon request, signs whatever unequal trade treaty the Germans present, then yes, the rationale for Germany fighting France is gone. On the other hand, if France doesn't do all of that, then Germany will have a number of reasons to still invade France. The biggest one being the loot (which Germany needs to maintain its blistering re-armament plans).
fasquardon