In 1942, in OTL Singapore fell on February 15 (see the Fall of Singapore). Yamashita's Japanese were nearly out of supply but by grit and bluff kept fighting until the Empire troops came apart. Even then as the Japanese later admitted had the British counterattacked they might well have driven the Japanese off the island. It was a classic case of one side with the will to victory versus another side who were defeated in their minds before the fighting began.
Singapore as Tobruk by Scott PalterThe double headed Allied problem was a total incomprehension by the Commonwealth forces of the Japanese style of warfare and the inept British commander, Arthur Percival (see Arthur Percival). Now there is no way white imperials were going to take Chinese advice on how to beat Japanese. The Chinese may not have had enough good units and supply to heed their own advice but had the basic concept of how to beat Japanese infiltration tactics down correctly. You forted up and let the Japanese run around your rear until they ran out of supply [which happened fairly quickly as they were essentially light infantry]. You then exterminated them.
So we will take the incomprehension of Japanese style warfare as a given. It took the US into early 43 to grasp this. It took the Empire under Slim about a year longer. However there was no reason Percival himself could not have been replaced. I will have Wavell give Singapore to Slim (see Middle East Campaign). Have Percival replaced before the retreat to Johore when it was clear he had lost Malaya to a numerically inferior force. A competent British commander beats Yamashita at Singapore and probably retakes at least southern Johore (see Geography) . Yamashita may have had air superiority and some tanks but he was essentially out of ammunition and nearly out of fuel.
So now the fun begins. The repulse at Singapore will slow down the Burma Campaign (see Conquest of Burma). Even a delay of a week gets the 17th Indian Division back across the Sittang River which in turn gives Alexander time to mount a defense of Rangoon.
Now there is no way the Commonwealth actually holds the Singapore-Sumatra complex. Japanese naval-air strength is too great. However launching the second strike at Singapore, taking longer to finish Sumatra and taking much longer to take Rangoon and from there Upper Burma mean the six carriers of the main Japanese Combined Fleet are not available for the Coral Sea, Midway or the Ceylon Raid. They are tied up making sure these operations come off well. The Japanese forward base on Guadalcanal is never built and no thrust is made to Port Mosby from Buna in New Guinea. The South and Southwest Pacific Campaigns as we know them never happen. The Japanese fleet is too strong and the Allied fleets are too weak.
So the New Guinea and Solomons theaters are scenes of land based air duels and commando raids but not a major combat sector for either side. The big sea battles come in early 1944 when the US Essex class is ready. The knock on effect of this is to enhance the North African campaign. The Allies were abysmally short of shipping and landing craft. Neither the Solomons nor New Guinea used all that many divisions but both were hogs of various types of shipping because of distance and because the lack of ports facilities meant local commanders would use them as floating warehouses. The ships that aren't going to the Pacific can help Ike do his logistic buildup faster in Algeria in the winter of 42-43. Tunis probably falls 45-60 days sooner.
In turn the extra amphibious lift means when Sicily deadlocks Ike has the sea lift to land a corps in the Italian toe to trap the Germans in Sicily. This in turn means a somewhat faster advance to what became the Gustav Line (see Salerno Landings) is also an easier landing. Note that this does not crack the Gustav Line any faster. The mix of terrain and German operational superiority preclude this. Essentially it took the massive Allied numerical and air superiority of the spring-summer of 1944 to break the Gustav and take Rome.
The big changes come in 1944. US is only supporting one Pacific campaign [Central Pacific to the Marianas]. That in turn is happening probably in fourth quarter instead of third [sea battles will be needed to attrit the Japanese Fleet]. This means that the two landings in France can happen at the same time instead of spaced apart by 2 months [the same shortage of landing craft in OTL]. This makes Normandy less bloody but the battle for France more so. With Sixth Army Group coming up the Rhone Valley Hitler's stand and die in Normandy gets ended much sooner but that leaves more good German troops to fight river line by river line across France and the Benelux. This hurts allied manpower [both Anglo powers were quite short of front line replacements by 1944 although the US is less bad off because the Pacific is using fewer ground troops] but helps the logistics [a somewhat slower advance means more of a chance to build railroads instead of relying purely on the Red Ball Express (see Red Ball Express).
In OTL we kept falsely believing we were almost at the point of a German collapse from mid-September of 1944 till the Ardennes in December. Here we know we are in a hard fight and are surprised when the German armies essentially implode in late January of 45. Hitler probably wastes the panzers he lost in the Ardennes in OTL relieving Budapest and then is left desperate from Stalin blows away the Vistula line. The German War ends two months early [although in about the same positions - that was almost baked in by a combination of geography and the predetermined occupation zones].
That still leaves the Pacific War. With the nukes still a maybe [the Trinity test is still in the future here] and Japan with some semblance of a battle fleet there is a possibility for negotiations. Given the fantasy world the Japanese higher military commanders lived in possibility is as far as one can go. The civilians wanted to quit after Saipan, ( see Battle of Saipan) , in OTL. Fear of a coup and / or murder of the civilian cabinet members delayed things for a year. However there is a chance for Japan to get a more limited occupation and be left with the Kuriles and Taiwan.