[BartSaM]

I really think that the humanitarian problem is not a real problem for China. A country this size could easily swallow 10 million Koreans (let's be real, surely not more than half of population would flee), allow them to work and assimilate and not even notice it really. I think the issue is that China is afraid that united Korea will become a US ally and allow US forces to station there. Also it could expose China to the world as a weak player that could not handle Korea by itself.

[opportune]

I've never understood this argument. South Korea is already a US ally, and I don't see how the land of North Korea is more strategically beneficial than South Korea already is. It's not like the US needs to be directly bordering China to be able to launch missiles there or to spy on them.

I think you are right that there is a "prestige" factor to this as well. China doesn't want to lose an ally, regardless of whether having that ally benefits them much, because they would look weak backing down to the US.

[BartSaM]

Land border attracts tensions. It's also a psychological factor. Then it is much harder to launch an invasion from the sea, than from land.

Its same as saying that there is no difference if Cuba will be "east" aligned and have USSR army/nukes or Mexico - after all, it is easy enough to launch nukes from both places.

You think from a "nuke" perspective. There are much more perspectives, like controlling water borders, locking important Chinese ports etc.