I don’t think China has any degree of control over NK personally. They don’t ‘use’ the North so much as opportunistically try to capitalise on the North’s actions when they can, while minimising risk of things getting out of control. They’d be very, very far from thrilled about the situation either.
The last thing China wants is a nuclear exchange between North Korea and anyone, given that NK would be devastated, and that would leave China with a massive plume of radioactive contamination drifting who-knows-where across their border and territorial waters. Quite aside from the contamination, there’d be a truly titanic panic and refugee problem, consisting both of Chinese from the contaminated areas and fleeing North Koreans. Unrest, panic, and tens of millions of refugees, many probably with severe radiation-related illness. The mind boggles.
China just wants to stop the NK situation getting too bad. They’re probably ok with the statuis quo tbh, or at least they were before Trump started sending out cranky toilet-tweets and everyone in the North Pacific basin started crashing nuclear exchange casualty estimation websites. China doesn’t really want a unified Korea because even though NK are ratbags, there’s a sense that reunion would mean final defeat for China in the Korean War. It’s a pride thing. And even South Korea doesn’t REALLY want reunion either, when they think about what it means.
There are 25 million North Koreans. That’s one for every two South Koreans. The great majority live in poverty unimaginable in the South, their infrastructure is primitive or decrepit, they are overwhelmingly malnourished and developmentally disadvantaged as a result (i think I heard a while back that average adult height in NK is under 5 feet, due to childhood malnutrition). I dread to think what sort of mental problems are endemic there, given the appalling oppression, starvation, indoctrination that has happened under all the Kims. These are, largely, not people who can contribute significantly to a modern economy, and who are woefully ill-equipped to participate in a democratic society. And there are 25 million of them. Trying to absorb that population would crush the South Korean economy. There’d be a huge influx of largely unskilled refugees into the south. There’d be ghettoes, riots, and it’d last decades. And do you give these people the vote, straight away, after they’ve been brainwashed into thinking the Kim family are demigods for the last 50 years? What does that do to the South’s democracy? Does a Kim end up getting elected? And it’s not like China is thrilled with the prospect of millions of North Koreans flooding over their borders if order collapses in Pyongyang either.
North Korea is as intractable a problem that exists in international politics right now. NK is nobody’s puppet or tool, but they’re everyone’s problem. China included.