Were there alternative policies that might have prevented or greatly reduced the death-toll of the Holocaust?   Sure. 

First, get the early Christian Fathers to lay-off anti-Semitism.  No anti-Semitism, nothing to infect Hitler’s mind a couple thousand years in the future (except anti-Communism and vegetarianism), so no Holocaust.[1] 

Second, occupy Germany for forty years after the First World War, same as after 1945. 

Third, open borders for the United States after 1924.  No Jews in Europe for Hitler to want dead, no Holocaust.[2]  On the one hand, the law relied upon a belief that citizens in a democracy had a right to control who entered their country.  This is parochial, rather than cosmopolitan. 

On the other hand, the 1924 law targeted immigration from Southern and Eastern Europe in particular.  No anti-Semitism (see above), no restrictions on Jews from these areas.  No anti-Catholicism, no restrictions on immigrants from Poland, Hungary, Italy, and Rumania.  Just get the Catholic Church to lay-off the “Syllabus of Errors,”[3] kidnapping Jewish children to force their conversion,[4] and maintaining separate schools.  Get the immigrants to stop undermining labor unions with workers eager for jobs and providing the votes for big-city “machines.”    

Fourth, tell the Arabs to lump it between the wars.  There were only 750,000 Palestinian Arabs.  Move in 7-8 million European settlers during the twenty years before the Second World War.[5]  Stamp out any Arab rebellion.[6]  The new settlers probably would be willing to help. 

While you’re at it, give the Italians Ethiopia and the Japanese China.  That way the British aren’t looking at fighting three simultaneous wars for which they do not have the resources.  In turn, the British wouldn’t have been so hot for appeasement.  They could have devoted themselves to winding up the French instead of encouraging weakness. 

Fifth, Britain, France, and the United States could have joined together to fight the Germans at the time of Munich in 1938.  If other people hadn’t been obsessing about how terrible had been the previous war, it might have been possible to stop the Germans before they got going.  The Czechs (certainly) and the Poles (probably) would have joined that fight.  The Americans should have abandoned their traditional policy of no foreign entanglements.  It might not even have come to a fight.  German elites might well have decided to toss Hitler overboard to avoid such a war.  Make him an offer he couldn’t refuse. 

Sixth, in 1939, Britain and France could have given the Russians what they wanted—control of Eastern Europe—to join an alliance against Germany.  Tell the Lithuanians, Latvians, and Estonians that the Russians are taking over.  Tell the Poles that they’re going to have to let the Red Army into their country to get at the Germans.  Tell them, as well, that the Russian price for “saving” them is the eastern third of Poland and Polish Communist in the government.[7] 

Other than that, you’re left with the situation as it really existed. 

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[1] That doesn’t mean that there would have been no Second World War.  All those non-refugee German scientists would have been working on a German atomic bomb to run with the V-2. 

[2] Still, Hitler came to power because of the political crisis arising from the Depression, not from anti-Semitism. 

[3] See: Syllabus of Errors – Wikipedia  The “Selected Propositions” rejected by the Church explains a lot. 

[4] David I. Kertzer, The Kidnapping of Edgardo Mortara (1998). 

[5] There are about 7 million Jewish Israelis today.    

[6] See: Great Syrian Revolt – Wikipedia and 1936–1939 Arab revolt in Palestine – Wikipedia 

[7] “Katyn” Katyn (2007) – massacre scene part 2/2 (English subtitles) – YouTube