The reason that alchemy could be performed on Earth apart from Amestris in the movie was that a homunculus was used which from creation had a relationship with the gate already; I assumed that the gate was "confused" and tried to pull both ways thus creating a tunnel or whatever. Then again though, in the anime it said that humanity developed on a different path in both worlds, so it could happen then in both worlds hypothetically. The energy for transmutation though is dependent on souls crossing into the gate though, so Amestris' pull on that could make it close to impossible. Then again, I do not really know.
The atomic bomb part would be hard to expand upon though because Japan really did get blown up by two of them. Unless the series broke off into another "universe" or something, it would be limited.