"Would Japan have won WW2 without the use of atomic weapons "? NO way, not even close!
What would have happened without the development of atomic weapons? In my opinion:
#1. The Soviet Union MAY have attacked Manchuria, and Korea with less vigor.
#2. The Soviet Union MAY have attacked with more vigor and gone vastly more distant.
#3. Not facing a bluff of an unlimited supply of atomic weapons the Japanese Emperor may have requested Gyokusai and every man, woman, and child met the Allied invasion forces with sharpened bamboo poles in suicidal attack in defense of the Homeland Islands.
#4. Japan had reserved all of the old wooden biplanes invisible to radar at that time, painted them black and loaded them with explosives to meet the Allied land invasion effort. Japan had developed and trained maned suicide plywood boats loaded with explosives powered by outboard motors to ram into the invasion fleet. Actively drilled small children to pop up out of small trenches loaded with explosives to dive under simulated tanks. Drilled school children in the use of bamboo poles to meet invasion forces. MacArthur's largest invasion force in history would be thrown back upon the sea.
#4. The American population would tire of the loss of life and cost of a Japanese land invasion and accepted less than 'Unconditional Surrender with Emperor subject to war crime trial'. War Bond sales boosted by the Rosenthal photo raising the flag over Iwo Jima would plummet to zero, initially funding the building of B-29 bombers and the bombs delivered to Japan, the B-29 would remain on base without bombs or fuel. Other nations would demand gold bullion only in exchange for trade from the USA.
#5. Japan could become as Cuba to the USA.. Japan in poverty would trade with other nations just as Cuba but not the USA.
Ultimately modern war is based on industrial strength and available natural resources. Japan in Aug. 1945 had nothing left but it's population. Six months prior Japanese last air freight to Iwo Jima contained not ammunition, medical supplies,water or food but (some) bundles of bamboo poles infuriating Iwo defenders. Both Germany and Japan had nuclear weapon development plans but totally failed production resources. The display of development of atomic weapons was a final slap of Japan's defeated face... and not much more. (secondary to the loss of Manchurian war material industries and raw materials to the Soviets)