The mistake you are making is the common one of reading history backwards, ie you are looking at early German actions through the prism of the Judeocide that did occur in historical reality, and concluding that since the actually implemented Judeocide had an exterminatory purpose, then all prior actions or alternative actions must have had a similar exterminatory purpose.And that's exactly why the Madagascar plan needs to be seen as exterminatory in the sense that massive death by attrition, neglect, and abuse was inseparable from the plan. Even if the Nazis didn't articulate it to themselves as an extermination plan it was only because they were so delusional and callous about it.
That conclusion ignores the fact that the Judeocide was resorted to in a specific context, that of a long total war that Germany had started to lose. It cannot be assumed that in a quite different context, eg an end to the war with Britain in 1940 and a negotiated peace with that country and France, any exterminatory action against the Jews would have been implemented.
An example is provided by the Nisko Plan , to which reference has been made. A discovery made in the former Soviet archives after the collapse of the Soviet Union has necessitated a fundamental reinterpretation of that action.
That discovery consisted of correspondence between Eichmann's office and the Soviet authorities responsible for immigration, in which Eichmann asked the Soviet office to permit the transfer to Soviet territory of all the Jews living in the German Zone of Occupation of Poland, under the agreement on population exchanges that had been part of the Borders and Friendship Treaty of 28 September 1939.
The Soviet authorities refused the German request, on the basis that the population exchange agreement applied only to ethnic Ukrainians and Belarusians living in the German Zone and ethnic Germans living in the Soviet Zone, not to Jews.
The discovered correspondence provides an explanation for the German plan to concentrate the Jews of its part of occupied Poland, and perhaps also the remaining Jews of the Greater Reich (Germany, Austria, Czechia) in the Lublin District, immediately adjacent to the border with the Soviet Zone. It was not so much to keep them there permanently as to concentrate them for the purpose of transferring them into Soviet territory.
It follows logically that the German intention in asking the Soviet Union to accept up to two million Jews cannot have been exterminatory, since if the Soviet authorities had accepted the German proposal, then those Jews would no longer have been under German control, and their fate would have been in Stalin's hands, not Hitler's.
If the German aim in relation to the Nisko Plan had been to decimate the Jews concentrated in the Lublin District by starvation and other means, Germany would have kept them firmly under its control rather than hand them over to another power that had no record of persecuting Jews, but rather tended to favour them.
Furthermore, the German aim of transferring the Jews of the regions under German control to Soviet territory explains why Nisko was selected as the location for a transit camp for Jews. Nisko is located on the San River, which under the original partition plan incorporated in the Non-Aggression Pact of 23 August 1939, was to be the border between the German and Soviet Zones of Occupation; accordingly, it was in a perfect position to hold Jews prior to their being transferred across the border into Soviet territory.
As is well known, the original partition plan was modified on Stalin's initiative in the Borders and Friendship treaty of 28 September, with the Zonal border being moved eastwards to the Bug River, transferring the Lublin District to the German Zone. However, in the four weeks between the beginning of the German invasion of Poland and 28 September, the German authorities acted on the assumption that the San River would be the border, which is why Heydrich's orders to concentrate the Jewish population envisaged their being concentrated to the east of Krakow, and Sipo Einsatzgruppen were very active along the San, rounding up the local Jews and pushing them across the river into the area that was to come under Soviet control.
Even after the line of demarcation was moved to the Bug, German units continued to push Jews across that river throughout October, November and early December, until the Soviet occupation authorities began to object and push the Jews back. It was probably the Soviet resistance to the unilateral German action of expelling Jews into the Soviet Zone that prompted Eichmann's office to write the two letters of early 1940 referred to above.
Once the border was moved from the San to the Bug, Nisko was no longer geographically suitable for a transit camp, but possibly policy inertia was at play here, with Eichmann hanging on to the location that he knew had been originally chosen, in his eagerness to begin the deportations from Vienna and Bohemia-Moravia on his own initiative. The shifting of the border may also have been the reason why Nisko was so quickly abandoned and the deported Jews held there returned to their homes in Vienna and Morava-Ostrava.
The bottom line is that if the German purpose in concentrating Jews in the Lublin District in late 1939 and early 1940 was to transfer them to Soviet control, then that purpose cannot possibly have had an exterminatory motivation. The retention of the concentrated Jews in the Lublin District, and everything that flowed from that, was obviously not the original German plan, but was a result of the failure to obtain Soviet agreement to allow their immigration to the Soviet Union.