Take the Korean war. If the South had indicated they wanted to join with the North the USA would have had a tough time stopping them. If the North had indicated they wanted to join with the South the Soviets would have had a hard time stopping them.
The rest of your post is basically saying that given the options the Koreans choose to evolve as two separate peoples with differing values and government philosophies. Sure. But that's not the west directly doing it, it is the west creating the conditions under which it could happen. The Koreans are the ones doing it.
the fight ensured that it was a far bigger, nastier, and longer war than it would have been otherwise
I don't know about longer. There are plenty of ethnic conflicts that have gone on for centuries in countries. I'd argue the bigger, nastier probably means shorter.
If either of the superpowers had stayed out of it, one or the other side would have won the war reasonably quickly.
I'm assuming if both superpowers had stayed out of it. Otherwise you are just talking about the conquest of at least a large chunk of Korea. If both stayed out of it, I'm not sure it would be over quickly. It might very well be that they couldn't unify. I suspect the South wins without interference at this point since the economy is so much larger.
Its also important to keep in mind that outsiders of all types have been fiddling around in Korea since the 1600s- first the Chinese, then an incident with the American-owned ship "General Sherman", then the Japanese, then the USSR and USA
That's the case with every country. All countries get fiddled with. Powerful countries spend a tremendous amount of their time managing the weaker ones. Weak ones tend to be proxy for a strong one. Middle power get stuck in the middle quite often between various powerful ones. That's not unique to Korea. I'm sure France would have liked to have not lost to Prussia then had a 1/3rd of their young men wiped out a generation and a half later by Germany and then be conquered in the next generation and then having their economy reorder for the next 2 in line with German priorities.
Given that history, it shouldn't be surprising that they turned inward and cut ties with the West and South Korea, which is strongly aligned with the USA.
They've done more than turn inward. They've been provoking the USA, South Korea and Japan for 60 years. If they wanted a quiet life of seclusion there wouldn't be nearly this much stress.