I don't recall exactly where I read it (it was some interview or something with the people who made the movie 10+ years ago), but I think it was a little more extreme than that. IIRC, (the perhaps unstated) bit of the backstory was domino theory was true and the US was badly on the wrong end of it. The NATO dissolving thing was just part of the picture.
It would have starved, if that were true. The Soviet Union was a grain importer for much of its existence.
"Couldn't conceive" is pretty strong - it was the era of the Cold War, GI Joe, a hangover from the Cuban Missile Crisis, the contras. Maybe accurate to say the average American would find it highly improbable, and our eventual victory always assured because Chuck Norris, Rambo, and the inherent virtue of our cause against totalitarianism.
Anyway, I think the paratroopers that appear in Red Dawn are from Cuba or a socialist country in Central/South America, right? Which isn't terribly far-fetched in the context of the eighties imagination - maybe more accessible idea than a Soviet D-Day-style invasion. Also not out of line in history - didn't the Germans court the Mexicans in WWII? The French during the Civil War (I use the word "court" loosely with the French 'intervention', but hell, we got a holiday out of it).
The film was marketed saying that no foreign invasion of US soil had happened, though it ignored the Japanese invasion of Alaska. The "average American" probably still doesn't think of Alaska very often except for reality TV, much less its history.
https://www.archaeology.org/issues/433-2107/letter-from/9780...
Italians and Germans were active all over the Americas, in response the US created the Office for Inter-American Affairs [0] to spread its own counter-propaganda. Among other things, they hired Disney to create The Grain That Built a Hemisphere, using corn as a symbol for Pan-American unity.
[0] https://en.wikipedia.org/wiki/Office_of_the_Coordinator_of_I...
Yes [1].
NATO being dissolved is pretty deterministic as well. The average American in 1984 couldn't conceive of the Russians actually making it all the way over to the US. Nuking the US, but not a successful land invasion. So there had to be some reason the US got weaker. NATO dissolving doesn't put any blame for that on the US, which is important for a flag-waving film.