Re: Follow the money
"At this point it's in China's interest to have the Koreas reunited as this would end the USA mandate to be on the peninsula and China's no longer desperately poor compared to SK"
The assumption is logical, but there are other reasons why China might not want that to happen. For one thing, if North Korea ceases to exist, nothing says the US would definitely take their forces away or that Seoul would ask them to. In fact, if they continue to have an alliance, it could mean that US forces end up moving to the north instead, putting them even closer to China's borders.
China does not have full control over what North Korea does, but it can encourage some activities and motivate Pyongyang to do what it says. This may be useful in a variety of ways. The typical example, and one that is plausible but only relevant if China changes their plans for Taiwan, is using North Korea as a distraction if they conduct a military operation. For instance, if they can get North Korea to start something when they invade Taiwan, then US, Korean, and Japanese forces would be divided, possibly diluting their ability to counter an invasion of Taiwan. There is also the fact that North Korea and Russia have closer diplomatic ties and that Russia doesn't really want to see North Korea cease to exist. North Korea has been playing Russia/Soviet Union and China off each other since it was founded, being sort of friendly to both while seeing who would give them the most, and they're likely to continue doing so.