Re: Worldbuilding-Adventure Friendly Cold War/Alternate Earth
 

You're going to need a pretty radical historical change to make China organised enough to take over even a mostly-defeated Japan. Deleting the Chinese Communists would only be a start: you'd need more to make it plausible.
It's rubber physics, but it is fairly subtle rubber physics. Understanding it requires knowing a bit about how nuclear reactions work:

When a uranium nucleus fissions, you get neutrons produced. Those can fission more nuclei, producing a chain reaction. Provided at least one neutron from each fission causes a further fission, the chain reaction continues. This condition is called "cititicality" - a critical reaction continues or accelerates, a sub-critical one dies away.

Real-world uranium produces two-and-a-bit neutrons on average, so if most of those cause further fissions, the number of fissions increases rapidly, and you get an explosion. This is an atom bomb

In a reactor, you arrange things so that about one neutron per fission causes a further fission. You arrange for the rest to be lost or absorbed. A reactor's control rods are absorbers. When you pull them out a bit, they absorb fewer neutrons, so the number of fissions increases, and the reactor generates more heat.

There is a subtlety in that not all the neutrons from a fission are released immediately. Most of them are, and those are called "prompt neutrons". But a few percent of them are released a little later, a few tens of milliseconds. that isn't long, but the prompt ones are released on a timescale of nanoseconds.

Those delayed neutrons are important for a reactor. If you arrange things so that the reactor is only just critical, because of the delayed neutrons, things happen slowly and can be controlled by humans. A bomb is critical on the prompt neutrons alone, and everything happens in a millisecond or so. You can't throttle it - it just goes off.

Now, if you want your world to have reactors, but not bombs, you tweak the physics so that there's only an average of 0.9 or so prompt neutrons per fission, and another 1.2 or so, on average, delayed neutrons. This is rubber physics because I can't tell you what else in physics would have to change if this was true. But it has the effect you want: it's absolutely impossible to build an atom bomb, but reactors are possible. As a bonus, they are easier to control.

Re: Worldbuilding-Adventure Friendly Cold War/Alternate Earth
 

You'd need to do something like give Sun Yat Sen a cast of Founding Fathers rather than just some devoted admirers.

Somebody needs to succeed him and somebody else needs to succeed the second guy and muiltiple somebodys need to create viable political parties to absorb the political energies of the country and invest them in stable government and peaceful changes of government.