You are trying to bring morality into a discussion about realpolitics - This is a straw man par excellence.
Unlike Nazi atrocities most of which were hidden/unknown from their own people and the Allies - everyone could see the senseless devastation of Atomic Bombing (assuming it was even possible against German AA defences). The Germans can build more underground factories and disperse the population from the cities. They can also shift troops and industry into occupied cities. Will the Wallies (assuming they can) eradicate Prague? Vienna? Pilsen? Paris? Rome? Oslo? Warsaw? Will they leave (Central) Europe as a nuclear wastleand (assuming they could) guaranteeing them the hate of the entire continent for generations to come? Will the public in the US/UK be supportive of atomic bombing once the 8th atomic bomb is dropped killing another 100 000 people without any effect? You might think that sitting on your ass comfortably at home, people that have been in this war for 5 or 7 years will think different.
Mutual exhaustion. As has been demonstrated time and again, Britain is crumbling by 1945. No manpower left. The Army is shrinking, the industrial workforces is shrinking, the war effort cannot be sustained any longer, or only in vastly reduced form. Even the Americans were beginning to feel (manpower) exhaustion by early 1945. Dont mind the economic situation, the psychological impact of a defeated Russia in 1943 and thereafter inactivity for two years in Europe. Dont mind the V2 bombardment of Britain. Dont mind the monumental task of defeating Japan, which will take well in 1946 without the Soviet Union, regarless of what approach is taken. At some point society has enough.
And you are ignoring manpower,casualties, public support and exhaustion.
You brought in morality by claiming that the ATL Allies would be more more evil than OTL Nazis while ignoring the ATL Nazis. And you listedf as one of the voting options "
The Wallies use dozens of Atomic Bombs during 1946-1948 killing dozens of Millions to win the war." Seems rather unfortunate, since it implies the Allies are doing it for the lols and not to stop the Nazis killing 100+ million people as well as freeing millions more from tyranny.
N, ,the morality is a very important part of it. For lack of a better phrase the nomenklatura of the US will demand Germany stopped. FDR is still president; how many of the leaders (and other influencers, like newspapers, radio, politicians, etc) were horrified by Kristallnacht? Men like Curtis Le May wouldn't care, but they will see that Germany will be only stronger as they exterminate enemies and consolidate, if given peace. Businessmen see the Nazis ruining the trade routes. All will see that germany will not honor any agreement. So, whom among the Allies will advocate peace? You ignore press time controls for the Allies, and forget the "
Why We Fight" series. The Allies won't use the same propaganda tactics as the Nazis, but since they are different political societies they don't need to. No, they would not target occupied cities, but target the Ruhr valley and use atomic bombs like in Operation Downfall, clearing out German armies for the breakthrough. You forget the US bombing of Rotterdam, quietly hidden (because the Dutch knew it was unintentional and not terror bombing like the German one), or the French civilians killed accidentally during the Normandy invasion. The world didn't condemn America since it knew it was accidental. Also, why is it then that the world- where the US announces the Germans exterminating whole races of people (using 1940 language)- would even know of the extent of US atomic bombings? And why would the US bomb cities outside of Germany? How many Germans would keep fighting as Germany is fully captured? If any non German cities are targeted, it would only be the railroad marshaling areas- still kill thousands, but what good are factories with no coal supplies arriving, and finished goods leaving? Your OP states the USSR is defeated end of 1942/early 1943. It does not state the USSR fractures into warring states like China in the 1920's, or that the people are so disillusioned like the Czar's armies that further resistance is impossible (and it is ASB for that to happen- the communists were too much in control for either situation to happen).
What would likely happen- The US is still taking North Africa by 1943. Germany can't move enough troops in. The Allies might take Sicily- it is an island, so Allied naval power provides them a way to cut off Axis supply. The Allies prob go ahead with Sicily- Germany has too much coastline to cover, the US has too many naval units to counter. Italy stays Axis since too many German soldiers. However, since there is no more land battles for 2-3 years, the Allies focus on the Pacific, focusing on a Central Pacific push, getting about 6-8 months ahead of OTL. US decides to stay with blockading Japan (knows needs soldiers for Europe, the USN is enough to keep Japan isolated and unable to interfere). OTl, for Operation Downfall the US expected to have 15 atomic bombs ready, plus the 2 used, meaning that Nov 1 1945 the US has 17 bombs ready. The Allies might invade in 1945, but 1946 is more likely. The Allies have continued bombing German cities to keep up pressure- Big week probably happens in 1945 with the same general results (Luftwaffe broken, but higher casualties for everyone). POD after Dieppe, so Germany still thinks ports are important, but Allies aiming for open beaches and no shingle sand. Normandy probably "only" gets 3 atomic bombs, France get the 1946 equivalent of OTL 1944. If Germany has any large army groups, expect them to get a few nukes a piece. OTL The US was worried about nukes being reverse engineered, so they use them in quick succession on German cities. Many German generals are aghast at the destruction (figure 3 at Normandy, 7-10 more in various targets in France, and 40-50 German cities) but the Nazis demand fighting the Allies that are now firmly entrenched in France. I give it 50/50 that the USSR gets back in the war Fall 1946, but 100% back in by spring 1947- too many armies pulled back from the Easter Front.
To make a TL;DR: USSR out end of 1942/early 1943. Communists still in charge, as it is too ASB for the USSR to collapse into warlord states or so defeated Germany can have a Versailles Treaty Peace (OTL they did't despite losing so many troops in 1941- it's ASB for Germany improves on its 1941 performance in 1942). This means although Hitler can remove most of the armies, enough needs to stay to be a credible threat to the USSR.
1942- Torch landing still happen, since before POD
1943- North Africa still cleared of the Axis- more tanks and troops mean nothing since having to sail so far to Africa while the Allied noose tightens with air and sea power. 50/50 Operation Husky goes on- German troops have to be deployed across all of Europe AND demobilized for the economy. Def more costly than OTL.
1944- since no invasion of the Italian mainland in 1943/no Overlord in 1944, more for the fight against Japan. US needs more victories, so focuses on a central Pacific thrust rather than 2 (the USN and MacArthur). This means the USN is 6-8 months ahead of schedule, so no nukes available for invasion, and the US knows from the intelligence briefings would be a bloodbath (even if Japan has only 50% of OTL lined up). OTL, only MacArthur want to go ahead with invasion- USN decided no, rest cooling to the idea. Blockading allows the US to redeploy to Europe.
1945- Luftwaffe broken in ATL Big Week, as although more fuel available for training, the combined bombing campaigns still force Germany into a war of attrition- which, as the weaker power, it cannot win. Allied chiefs determine that due to troop/ship redeployment schedules and wanting to beat down the Luftwaffe more, D-Day will be Spring 1946 (prob determined in 1943, confirmed in 1944, but the lateness of Big Week confirms no "bouncing" France with resources on hand).
1946 late winter/early spring- Allies do ATL of wrecking France's transportation system, as well as USN/RN raids along the coasts of Italy, Norway, and Greece.
1946 Spring- Allies launch D-Day, using 3 atomic bombs to clear the Normandy area (no drivers for the Germans to focus on improving it, while Calais *is* a beast to take ATL). US also uses 10-15 atomic bombs at various targets in France, trying to minimize French casualties while cripple the Wehrmacht (so no nuke use on Paris). The near simultaneous invasion at Normandy and Southern France, coupled with the previous nuclear targets, means the Allies are entrenched on the mainland. 30-40 bombs used on the Ruhr valley, both to cripple arms productions and wreck train hubs. Despite Germany moving troops, the Allies grind to within range of the French-German border. The Spring 1947 offensive, heralded by the use of 30 atomic bombs, also has the Soviets launching a surprise attack against the Germans. By the end of 1947, Germany is defeated.
