Seems to me there's one extremely likely outcome that's being underestimated, because we know what the effect was: nuclear war. We know what the Bomb would do to a city, because it was used. TTL, they wouldn't. And TTL, even tho Stalin knows the power of the Trinity test, that's only a test, & really says nothing about effects on an actual city, which is why Hiroshima & Nagasaki were selected: they'd remained untouched so far (in part to serve as potential trial targets). Moreover, Stalin had doubts (FWI read, somewhere, once) the U.S. would actually use it on civilians (read: Soviet cities). I'm fairly sure, if he built the Bomb postwar, as OTL, he'd have no such scruples, & might find the U.S. didn't, either--the hard way. Can you say "Berlin Blockade"?
There's something else, too: the Bomb is damned expensive & technologically sophisticated; without demonstrable need, be it nuke-armed Nazis or Sovs, would anybody build more? Would Stalin even bother? Would Britain? Would there be an infernal rush postwar, or a more leisurely, "wait til it won't bankrupt us"?
And let me suggest one other possibility: without the "
Hindenburg effect" of Hiroshima/Nagasaki, could nuclear power become more acceptible? USG-subsidized nuclear merchantmen, in the fashion of
NS Savannah, say? Or NS
United States? NS
Queen Elizabeth 2?