1) You build elevated tracks for a portion of the way for a RR or road. Fits in well with the Autobann system. You can also give the Poles extra funds to improve their transportation system as a purchase price for the land. Add in some provision that Poland can also send seal trains to Hamburg and some docks in Hamburg, and it probably seems like a pretty good deal. Or for example, just tariff free inspectable trains through Germany to other nations.
If Germany could be counted upon not to obstruct Polish traffic in any way (a big if, and even if one regime could, there'd be no guarantee that its successors would be equally magnimous, but that's another discussion) the corridor would indeed have had no vital role at the time. But while not invaluable, it was certainly something very much worth keeping anyway. Because the time might have come again when Germany could not grab it - wether due to physical weakness, or distraction, or unwillingness to expend the blood and wealth required. Without the corridor Poland would still have been at Germany's mercy economically during such times, for even if Germany lost the stomach to attack, defending its own territory would have been quite another matter. But if Poland retained the corridor, it could have been used to gain a reprieve. Even a brief one could have done wonders. Ceding it only because it was of less use at the time then a road to Hamburg would have been extremely shortsighted. (Which is not to say that no good reasons could have existed - averting defeat, a fourth partition, and years of hostile German occupation would have been worth handing over such a long-term asset.)
2) Your 90% figure is probably right for villages, but not for the whole area.
3) A rearmed Germany gives Poland a strong enough ally to deter the Soviet Union from taking back the land in the east. Danzig was mostly German. We are talking about losing less than 10% of West Prussia. Add in a good diplomatic effort to help calm fears in France and UK, and we have a workable deal that will appear to secure Poland's future.
I was originally answering Simon Oliver Lockwood's suggestion that a return to 1914 borders with unspecified compensations for Poland in the east would have made a potential Polish-German alliance more palatable.
4) The only possible way to ensure German does not attack is an alliance with Stalin which will cost land in the east. The only way to stop Stalin in the long-term is a deal with Germany. Poland took the third rail, which was to make sure both were hostile. It is a matter of the leadership of Poland attempting to find a path that works, instead of emotional decision making. This dilemma was clearly understood by 1915.
Do you mean the specific situation which existed in OTL in 1939, or attempting to steer clear of both Germany and the USSR for as long as was practical in general?
(The Soviet option was not much of an option at all. The assumption that eastern Poland would have been enough to satisfy Stalin is questionable. In the short term - maybe. But if such a deal somehow did happen, the area between the Vistula and the Bug would become the new eastern Poland, and I guess Stalin would be watching it hungrily. There were very few Ukrainians or Belorussians, but Stalin didn't mind. The original version of the Ribbentrop-Molotov Pact divided Poland along the Vistula. It was later amended, but shows that Stalin was perfectly willing to expand west of the Curzon Line.)
From Polish POV both sides were more or less hostile. And after the fall of Czechoslovakia nobody trusted Hitler's promises anymore. Theoretically Poles didn't choose so poorly. Nobody could have predicted that rabid anti-communist Hitler would ally with rabid anti-fascist Stalin - it was a complete surprise for everybody and the deal was made only few day before the war started, when it was too late to change the policy. In early 1939 Stalin wasn't considered that much of the threat, since he sat more or less quietly and even agreed to negotiate with western allies. Poland gained support of two European powers against Hitler so in Polish opinion chances to avoid the conflict altogether weren't that bad (which was what Poland wanted in first place).
And even in case of war, France and Britain promised quick help. Poles believed that they might be able to hold long enough to give their allies necessary time to mobilize and launch their offensive. They underestimated German Army and overestimated their own capabilites, but that mistake was repeatedly made until 1941 by virtually every country fighting the Germans. Had the Poles been able to resist longer and without Soviet invasion and had the allies launched their offensive as promised (falsely, but Poles couldn't have known that) the result of war might be very different.
Yes, the Poles' error was their assumption that Hitler was an opportunist, and wouldn't have dared to attack anyone if this were to mean a war with France and Britain - and with this assumption their behavior did make a certain amount of sense. But while Hitler only conclusively disproved it in September 1939, I think the signs were there - Germany was still arming like mad and out on a huge annexation spree. Despite Hitler's magnificently unsubtle diplomacy, it would still have made sense to let Danzig go to Germany - the stakes were so high that giving him this one final chance would IMO have been warranted.