Your hypothesis seems difficult to support. If this were true, why then the (re)taking of the Saar region, Austria, and Czechoslavakia (twice)? How then to explain Hitlers declarations he was going to do these very atrocities (gassing of Jews, and Lebensraum) in Mein Kampf, written in 1925 (vol. 1) and 1926 (vol. 2)? This was written well before Hitler rose to power in Germany and seems to be before these supposed killing of German nationals by the Poles.
The Saar was part of Germany, but was placed under French occupation after WWI. Hitler’s motive for restoring it to German rule was probably much the same as would have been the case for the leader of any nation in that situation. The annexation of Austria was done to strengthen Germany, and because of the theory that Europe’s Germans should live within a Greater Germany.
Czechoslovakia is a more interesting case. The Sudetenland was part of Germany, but had been placed under Czech occupation after WWI. Hitler’s goal for reclaiming that was similar to his reason for wanting to reclaim the Saar. As for why he annexed the rest of Czechoslovakia: I imagine he had three motives. Czechoslovakia had signed a defensive alliance with the Soviet Union back in 1935. Annexing this nation sent a clear message to the leaders of other Eastern European nations who might otherwise have been tempted to take the Soviet side in the cold war between Germany and the Soviet Union. The second reason may have been to prevent Soviet troops from beginning any invasion of Germany on Czech soil. The third reason was probably to strengthen Germany and gain access to additional industrial capacity, labor, and raw materials.
During WWI, Britain and France had imposed a food blockade on Germany; which resulted in about 750,000 or more hunger-related deaths in Germany and Austria. The blockade continued on into 1919 to force the German government to sign the Versailles Treaty. Due to the reparations payments and economic damage associated with the Versailles Treaty, the Weimar Republic did not always have the money required to purchase food imports. The concept behind Lebensraum was to conquer enough Soviet land so that Germany could feed itself, regardless of economic circumstances or the possible presence of an Allied food blockade.
You mentioned that Hitler had written about his plans to gas the Jews in Mein Kampf. I’ve heard that idea thrown around a lot. But the one time I saw an actual quote from Mein Kampf in support of that view, it wasn’t nearly as convincing as I’d expected. Moreover,
While Hitler made several references to killing Jews, both in his early writings (Mein Kampf) and in various speeches during the 1930s, it is fairly certain that the Nazis had no operative plan for the systematic annihilation of the Jews before 1941. The decision on the systematic murder of the Jews was apparently made in the late winter or the early spring of 1941 in conjunction with the decision to invade the Soviet Union.
The above quote is from a Jewish website. While I have not examined this website enough to ascertain its overall level of credibility, their acknowledgement that there was no plan to exterminate the Jews prior to '41 is important. (And dovetails with what other sources have stated; though Tooze suggests that the extermination plan was not created until '42. Tooze also points out that Germany was in the midst of a food crisis caused by the British food blockade; and that killing the Jews was seen as a way of ensuring there were fewer mouths to feed.)
Prior to WWII, Hitler’s plan for solving the so-called “Jewish problem” was through emigration. Due to Muslim and Palestinian unhappiness with the number of German Jews immigrating to Palestine, Britain responded by eliminating almost all further Jewish immigration into Palestine.