A separate Q is “could we have implemented the NPT without Hiroshima, if scientists had strong evidence it would ignite the atmosphere?” People can have reasonable disagreements here; I think it’s lame not to try.
The unfortunate answer is likely not, assuming the cold war happens like it did historically. Both sides were very much going to get nuclear weapons and escalate as soon as they were able to. You really need almost Alien Space Bats or random quantum events to prevent the historical outcome of several states getting nuclear weapons. Now w imagine those nuclear weapons were intelligent and misaligned, and the world probably goes up in flames. Not assuredly, but well over 50% probability per year.
The unfortunate answer is likely not, assuming the cold war happens like it did historically. Both sides were very much going to get nuclear weapons and escalate as soon as they were able to. You really need almost Alien Space Bats or random quantum events to prevent the historical outcome of several states getting nuclear weapons. Now w imagine those nuclear weapons were intelligent and misaligned, and the world probably goes up in flames. Not assuredly, but well over 50% probability per year.