It is not hard to understand. It is not that China "wants to keep DPRK in power". Rather, it is China's view that it is no one's business outside the DPRK how the DPRK conducts it's affairs. This is not new or unique, it is how basically all of China's foreign-policy doctrine - keep your nose out of other countrie's business.
China never wants to be involved in other countries' problems nor do they seek to impose their will on other countries - you don't see China out trying to spread their own unique brand of communist/capitalism elsewhere do you? That's because they don't - China keeps to themselves, for good or bad.
It is hard for people in the West to believe this because in the West foreign policy is essentially *ALL ABOUT* spreading your influence and trying to spread democracy. China has no interest in any of this.
China is a regional power, but the reason they don't engage in the actions you discuss is because they currently are not a superpower. It's true that they are clearly "communist" in name only as they've clearly abandoned any notion of being part of an international socialist movement. That being said, their harassment of Japan, their cold relations with South Korea and their continued insistence that Taiwan/Republic of China is their property shows they don't mind badgering others, even if they're indifferent to the ideology of other nations.
They do indeed try to impose their will on other countries well outside of Asia in some cases too. A clear example of this is how they demand that every nation recognize the PRC as the "one China" which is why most of the Western world including America only have limited relations with Taiwan and those nations cannot refer to Taiwan as the "Republic of China". They're pretty fickle about how other nations relate with Tibet too. They manipulate the currencies of other nations, particularly the US Dollar to give themselves a trade advantage (though this is hardly unique to China, most central banks seem to do this including America's own Federal Reserve and their "peg" has been much softer in recent years).
I see you clearly want to imply there's something inherently "bad" about America playing the role of a superpower but you must ask yourself what is the alternative? When I look at world history, America seems like a far more gentle superpower compared with historical superpowers in Greece, Rome, the Khans, China, or Napoleonic France. I suppose British Empire wasn't *so* bad, though they still had much more desire for conquest and colonization than modern America does and America's concern for human rights isn't perfect, but it is generally better than that of Imperial Britain. What about no superpower? Historically, that seems to have led to instability and war. The various European wars and especially the two World Wars were caused by nations feeling they had a shot at winning because things were basically balanced. The Cold War and all the proxy wars it entailed were caused by the USA and USSR each thinking it could gain the upper hand.
I know Westerners are raised with the notion that being "fair" is always inherently a good thing, but the truth is, sometimes an unfair balance of power can promote peace. America's lopsided power advantage over other nations has led to most nations having little desire to want to fight America because they know they'd loose. Likewise, America doesn't need to fight most nations since most will concede to them. The fact that American voters don't generally have much of a taste for conquest or long wars generally keeps America from demanding too much. It's not "fair" but it does promote peace.