I have a few critiques not worth full answers, but you could edit them into your narrative - 3 billion people are killed in 1991 in a war between two countries, the combined population of which at that time is 397.9 million. This is assuming every person in each country dies, so maybe 50 million would be more accurate. Finally Unpredictable Continental Drift could use some elaboration, that's sort of a big thing to occur for unknown means. Otherwise looks great

The 3 billion was accounting for everyone else that died afterwards, but you're right, im reducing the overnight deaths from 120M to 50M.

3 billion out of 5 billion people in the world (60%) is a lot if only 13.1% world's land surface area is in countries that were hit, and a maximum of 50% of the landmass of each country was hit effectively. That means that you hit 6.55% of the world's land to kill 60% of it's people.

I thought that radiation and fallout would spread a lot, especially when there's big hydrogen bombs involved. And then, the statistics would also take in account indirect deaths caused by the nuclear war.

Radiation would spread best over land so you could consider killing off half of the population of every country adjacent to Russia or the US. Remember that the more people you kill off, the more time subsequent developments in tech, etc will take because of a) cleanup cost of the bodies and destruction and b) less minds to do the inventing :P

Hm... I think I should come up with a reason to nuke all those 'innocent' countries...

After FDR, The US President can only serve for two four-year terms, so Kennedy couldn't be in office in 1971, unless the Twenty-second amendment never happened in your world.

You've killed off a large chunk of the world's population, there's food shortages, anarchy, etc. And you think people are going to start building rockets and blasting off into space? Not Gonna Happen. More likely you'll get some sort of quasi-religious dictatorship offering a quick-fix answer to all problems. (Mostly it's a way for the leaders to get laid a lot - see "If This Goes On..." by Robert Heinlein). Almost certainly they'll toss more nukes. (Once the "we can't do that!" wears off it becomes easy to kill millions at a time, as long as they're far away). Mine shaft gap, anyone?

@DarkestofNights - which Kennedy? John, followed by Bobby, followed by Ted, followed by..?

There weren't any Norwegian troops to defend Norway. The Germans knew they could not trust them. There were Norwegians in the Waffen-SS, but they were busy on the Eastern Front.

@BobJarvis I'm assuming JFK was never assassinated. If he was, then LBJ should be in office until '69, when Republicans took over.

@DarkestofNights - then in '68 Bobby would have run, and if we assume he wasn't assassinated I think it's fair to say he would have won, and would likely have been re-elected in '72. (Incumbents tend to be re-elected). In '76 Teddy would probably have run on his brother's coat-tails, and won, repeating in '80. So we'd have "President Kennedy" from 1960 through 1984. And post-Teddy I suspect the Kennedy clan would have supplied further candidates (sorry - I'm not up on my Kennedy genealogy :-). Or maybe Joe wouldn't have died in WWII and... (Not my idea - cribbed from Robert A. Heinlein :-).

Unpredictable continental drift - I can imagine the scenario: "Mr. President! The continental plates have sped up their rate of drift TEN-FOLD!" "My God! What does this mean?" "Europe and North America are now RACING APART!!" "HOW FAST, MAN? HOW FAST?!?" "At this rate, Mr. President, London and New York will be 150 FEET FURTHER APART in only 100 YEARS!!!" "((blink-blink)) 100 years?" "That's right, Mr. President". "So...I won't be in office, right?" "Correct, sir." "Uh...meh. What's for lunch..?"

I guess at this time, I can transform the reason to send people into space from "evacuating earth" to "showing who's the Good Guys and which faction have the best interests of its people". And after the destruction of the earth, Most likely, you can make more money in space than on the earth. Since the space infrastructure was already there, it's not unthinkable that a lot of factions would try and build the 'best' colony.

An alternate option to nuclear war might be something like: Alter WW2 history. America does not drop bombs on Japan for "reasons". This reduces public outcry/negative opinion of nuclear weapons. At some point, the SU jumps on the en.wikipedia.org/wiki/Project_Orion_(nuclear_propulsion) bandwagon and nukes their way to space. America and China join in to keep in the race. By the time they all realize they're destroying the planet, it's too late and all they can do is push the evacuation using their current nuclear propulsion infrastructure.

@BobJarvis Another option is that John could have two non-consecutive terms, like Cleveland. Either way, it seems like a thing worth explaining.

One unrealistic aspect not mentioned in the answers, and not important enough to warrant its own answer: you assigned rioting and anarchy orders of magnitude higher death tolls than the war itself. Unlikely to happen. Yes, there would be rioting and anarchy, but the death tolls will be not even close to the rations you listed. There were many examples of local apocalypses (many of them in WW2, with cities under siege for months or even years, without food, etc,) and mass starvations, but rioting itself didn't get even close to the highest death toll.

@Bloc97 This reminds me of the story of the Earth 2140/2150 series of games. Maybe you will get some ideas from it.

Hats off to the OP for putting some good thought into this.

Your timeline for development of space infrastructure is ludicrously optimistic. You go from first men on the moon to lunar base in 7 years, and then launching 411 million people into space in just another 18.

@Bloc97 Crucial problem is that there's virtually no amount of damage to Earth that would make it less inhabitable that anything else in Solar System. Maybe even in this part of galaxy.