Too optymistic.Japan and maybe England would survive,but France not,becouse soviets would keep throwing tanks at them till they fall,and they have big commie fifth column,too.
More interesting is what 1949 USA do in 1849 - go for conqest of the world,or sleep in splendid isolation?
Or maybe "only" care about making South America USA colony?
I would agree on the 1st point definitely. Britain didn't get the bomb until
Operation Hurricane in October 1952 and that was a test device. In a crisis such as a Soviet invasion it would become an urgent priority but there would be a hell of a lot of others, including a massive air and land programme needed to try and equip allies while they last.
Also the disappearance of the US means no Marshall Plan and a serious disruption of world trade. Apart from anything else the more important agricultural region in the world has just been lost at a point while Europe is still in recovery. Plus a lot of the equipment of the orphaned US forces and their allies, including Britain, is from US factories and their no longer there.
If US forces and especially nukes outside the US don't deter Stalin then the only other thing that might work could be massive gas attacks. Britain still have a large strategic bomber force and could do a lot of damage to targets it could reach. Whether and how quickly the British government decided to do that would be a big issue. Plus how quickly did the UK get rid of their wartime gas stockpile and did we have access to enough details to produce nerve gas in significant amounts?
As such if Stalin went to war and we can't decapitate the USSR I think Soviet forces would take most/all of continental Europe and probably the ME as well. You might be able to stop them on the Pyrenees, in Italy and at the Suez Canal say. They might get stopped in S Asia if India and Pakistan could get their act together given the logistical problems the Soviets would have, the numbers the defenders could assemble and possibly by then Britain and Canada could send significant aid. China would go communist and Korea fall but hopefully Japan could be held although that would mean a deal with the former Japanese military. How much of SE Asia would also fall I don't know.
If could be that an already weakened USSR would overstretch itself and collapse but I wouldn't rely on it.
1849 US is probably going to be largely ignored simply because the bulk of their neighbours are too busy with the Soviet threat. Although possibly Mexico might ask questions about California, Texas and the lands in between which to the down time US their only formally claimed a few years before. Up time US forces would oppose this of course but they might not have any resources available to do anything about this.
The other issue with 1849 US would be slavery of course and also if it does try any expansion of either its territory or slavery then it would very likely get stamped on. Definitely not going north and trying to introduce slavery into the up-time US Virgin Islands, or any neighbouring region isn't going to be accepted.
In terms of 1949 US in 1849 its going to see a lot of disruption due to lost trade and its also lost its overseas people, including those in places such as Hawaii which haven't been brought back. However once it adjusts, which shouldn't take long the world is pretty much its oyster as no one can oppose it militarily. Likely to seek to suppress slavery in the Americas but would it seek to do that in other areas, such as Africa and the ME? It might argue for the ending of European colonization but does that mean its leaving areas unprotected or stepping in itself and also what does it do about European settlements already in place?