Is the existence of absolute power itself not a restriction
I just don’t see how dialectical materialism is consistent with unchallenged and unjustified power.

Well there's your problem. Did you even read the rest of what Cowbee said in this thread, explaining much of how governance in China actually works? You come here basing your questions around this false assumption of "the existence of absolute power," when no one in China has absolute power, rather power is vastly more evenly distributed there than in liberal "democracies."

The fact that there is a single party is not (as western propaganda would have you believe) evidence of "dictatorship," but instead functions as a bulwark preventing reaction and the destruction of the revolution by capital - something I would hope you would be able to recognize even with a very basic understanding of dialectics. There is no reason the will of the people can't be enacted via a single party that exists to ensure it is their will and not that of capital that rules, indeed it makes more sense to have a single party when the rule of the people is the goal.

Consider how the approval rating for their government across the population of China, well over a billion people, is above 90%! And now consider the U.S. with it's "two party" system, where both parties represent the interests only of the political donor class (capital) and the government is largely despised by the population. The power there is concentrated in a small number of ultrawealthy bourgeoisie and it is continuously getting worse, more and more concentrated, while the people of the US are losing more and more of their so-called rights every day.

Yet you frame your questions under this base (and false) assumption of "unchallenged and unjustified power" in China without even considering how power is constantly challenged there (see Cowbee's explanation further up of the many direct elections in China) and through that challenge, its justification is consistently being reestablished.