I am thinking I want to set Hitler's "difficulty level" at "High" but also set his command experience level similarly at High, and I have no particular commitment to helping his side win, or making the world a better place by making his side lose faster.
So let's say that it Hitler's consciousness from February 15th, 1943 that comes back in time. He's run the Third Reich state for almost ten years, a war with some stunning early successes for about three years, but he currently smarting and bitter over the recent defeat and surrender of "Field Marshal" Von Paulus to the Soviet Untermenschen at Stalingrad.
He goes back in time to the Germany of November 15th, 1917. In this timeframe, Germany's west front has just survived the BEF's battering campaign of Passchendaele in Flanders, ended on November 10th. The French, by contrast have been out of action offensively for most of the year since the failed Nivelle offensive. America has been in a state of war with Germany officially since the spring, but American soldiers have made basically no impact on the ground, with American troop battle casualties by this date amounting to three individuals. The American Navy has made a greater contribution to convoys and the blockade at sea and the counter-submarine fight, which the Allies are defanging after the worst times of the summer of 1917. About a week prior to Hitler's arrival, the British announced their support for a Jewish National Home in Palestine with the release Balfour Declaration in the international press. And about a week earlier the Bolsheviks seized power in Petrograd from the feeble Provisional Government, followed soon after by Russia's other cities. Italy is reeling from its defeat at Caporetto in October.
To exercise maximum control over Germany's wartime administration, strategy, and military plans and operations from this point, who should Hitler drop into, Hindenburg, Ludendorff, the Kaiser? I would suggest almost certainly not Catholic Centre Party Chancellor Georg Von Hertling
en.wikipedia.org/wiki/Georg_von_Hertling.
Hitler-in-charge's scope of action and options are limited, but real, at this point. He can direct the the scope scope and focus of the spring 1918 offensive(s), govern home front resource allocation and rationing policy, the military policing, dissent monitoring, restraint, and punishment system, and take what he considers preemptive action against those he considers "the November Criminals" and internal enemies. He can also guide policies toward occupied peoples, minorities like Jews and Gypsies, political groups like Socialists, policies towards the other Central Powers, the secondary fronts like Italy, the Balkans and Middle East, and whether and what kind of peace to make with the Bolsheviks and what stance to take toward the Russian Civil War.
stevep , regarding structural restraint on anti-semitic policies, I have doubts these restraints will be absolute or would have the same causes you suggest. Withdrawing German Jewish soldiers and essential workers from service would be difficult and raise questions and cause on the ground resistance and noncompliance, but state media could start amplifying antisemitic themes ignoring wartime contrary evidence, using things like Judeo-Bolshevik guilt by association and Judeo-Zionist-British guilt by association exploiting the Balfour Declaration to play up Jewish threats and conspiracies. Even if German subjects/citizens are initially exempt from direct targeting, this could result in targeting by German occupation authorities and German client ethnic regimes in the east in the Pale of Settlement like the Baltics, Poland, Ukraine, plus occupied Romania and the Ottoman Empire adopting anti-semitic policies and anti-Jewish propaganda as part of their nationalist rallying late in the war.
I disagree with Fritz Haber being a reason for Germany to hold back on this. He was a scientist and technician, and others were widely using his inventions and knowledgeable of his formulae by late 1917. He'd be personally unmolested but wouldn't be likely to cast a wider protective net over European or German Jewry. The most formidable Jew in the Reich bureaucracy would probably be Walter Rathenau, but even he could likely be overcome in matters concerning 'ootjuden' or the Jews of countries to the east of Germany or those with 'traitorous' Zionist associations.
- or a war extending into early 1919 with an invasion of at least parts of Germany by the allies.