I have now obtained the book by Kessler, published in 1923.
Its title is "Germany and Europe", and was published in New Haven for the Institute of Politics by the Yale University Press. It consists of lectures delivered by Kessler at Williamstown, Massachusetts, in July and August, 1923.
The passage in which he refers to his negotiations with Pilsudski in Warsaw in November 1918 is on page 33 (with my emphases):
The reason adduced for Dantzig and Western Prussia, the necessity of giving Poland a free access to the sea, was a very poor excuse. Fir in the case of Czecho-Slovakia, which is just as much in need of a free access to the sea as Poland, and indeed more so, being a more civilized and more highly developed industrial nation, the difficulty was solved without any transfer of population or territory by the internationalization of the Elbe and a free port under Czech administration in Hamburg; and the Vistula, which flows through Poland to Dantzig, is a river nearly a mile wide as high as Warsaw, and could easily have been so canalized as to carry the sea up to the very Polish capital. That is what I myself, who was at the time German Minister to Poland, was prepared, on instructions of my government, to propose or to accept. A right of way on land, coupled with a railway neutralized and guaranteed by international agreement and a port on the Baltic next to Dantzig, under Polish sovereignty, could have completed the arrangement and given Poland a perfectly free and direct access to the sea. The Poles themselves would have been content with this. For Marshal Pilsudski, the Polish Chief of State, and a man who more than any other was responsible for the rebirth of Poland, when I spoke with him about Western Prussia, repeatedly made a point of asserting that Poland did not care about it, and that it did not seem to him in her interest that she should get it, although she might be forced to accept it if it were offered her by the Allies. Indeed, it was not the interest of Poland and not the necessity of securing for her a free access to the sea that forced the framers of the Treaty to repudiate the principle of self-determination in the case of Western Prussia, but the interest of France and the policy of France of dismembering Germany by cutting off her eastern provinces and tearing out by the roots every possibility of a friendly understanding between her and Poland.
Note that Kessler's concern is solely with West Prussia and Dantzig; the Posen Province is not mentioned at all, since it did not form part of his negotiations with Pilsudski in Warsaw in November 1918. That was because, even before the armistice 0f 11 November, the new moderate government in Berlin had accepted Wilson's 14 points, and hence that most of the Posen Province would have to be ceded to the resurrected Polish sate.
Indeed, between the armistice and the outbreak of the uprising toward the end of December 1918, the German Republican Government had recognised the mor4e moderate representatives of the Polish majority population in the Posen Province, and was engaged in negotiations with them on a peaceful resolution of the issue of sovereignty. Thus, there was no need whatever for a Polish nationalist uprising in the Posen Province.
The uprising was a result of Paderewski's visit to Poznan, in which he incited nationalist passions, and was instigated by the National Democratic Movement through various activist groups associated with it, such as Sokol. Its purpose was not so much to gain independence from Germany, which was already in process of being achieved through the terms of the armistice, but to drive out, through terrorist violence, as much of the ethnic German population as possible, in order to create a fait accompli bolstering the claim of the Polish National Committee to all German territory east of the Oder.
It is noteworthy that the uprising was immediately supported by France, which pushed for the Entente to recognise the insurgent leaders as the de facto government of the Posen Province, replacing the German administration.
It is also noteworthy that the line of demarcation between the area under the control of the insurgents and that remaining with Germany, decided on by the Allies, left Pomerania, West Prussia and East Prussia with Germany. It had not yet been decided whether those territories would be taken from Germany.
Kessler is correct when he sees France as the main force pushing for the excision of West Prussia and Danzig from Germany, for the purpose of both weakening Germany and creating ongoing hostilty between Germany and Poland.
He is also correct in seeing Pilsudski as not wanting to take West Prussia from Germany, because it would create Polish-German hostility.
However, he is incorrect in seeing PIlsudski's attitude as that of all Poles, and in ignoring the anti-German or Piastist tendency in Polish nationalism represented in particular by Dmowski, which promoted westward expansion by seizing German territory, and was opposed to the anti-Russian or Jagiellonian tendency represented by Pilsudski, which wanted eastward expansion and was less interested in taking German territory.
It was a combination of the desire of the Piastist nationalists to expand to the west, and the French aim of weakening Germany and promoting hostility between Germany and Poland, that prevented the negotiations between Kessler and Pilsudski from being realised. If the settlement described by Kessler had been achieved, and there had been no ethnic cleansing of ethnic Germans in the Posen Province in the winter of 1918-19, there would have been no cause for ongoing hostility between Germany and Poland and no cooperation between Germany and Soviet Russia, no "spirit of Rapallo".
If friendship between Germany and Poland had been achieved at the end of 1918, on the basis of the Kessler-Pilsudski negotiations, Poland would have had the support of Germany in its confrontation with Soviet Russia in 1919-20, and it is possible that the expansionist Bolshevik regime could have been overthrown at that time. If that had happened, it is most probable that Poland would have been spared the fate it suffered during and after the Second World War.