I'll start from the bottom and work my way up.

We don't unconditionally support Russia and China, we critically support them.

If they become imperialist in the future, we'll deal with it then.

China has no plans, nor the necessary superstructure or class relations to be imperialist.

The world is shifting to multipolarity, not Chinese unipolarity. Even so, geopolitical power is not immediately imperialism.

China is investing in projects around the world for multiple reasons, and yes some are out of self interest. That doesn't make the projects invalid in their use as local economic drivers, means to increase living standards, means to increase the power of the global proletariat, means to help countries develop in the way they themselves want to develop.

Engaging with these hypotheticals is a waste of time. It's like spending all day worrying about an extraterrestrial invasion or a meteor wiping out civilization. That's just not the world we live in today, and though it is a possibility it is so remote at the moment that it's not worth investing any effort into planning for the eventuality. We will deal with that situation if and when it happens but for now we should be focused on understanding the world as it is now, and acting according to the objective realities of the present and foreseeable future, not according to some fantasy scenario we imagine in our heads or some far distant future that may or may not happen.

Russia and China at the moment are not imperialist and the geopolitical conditions currently are such that there is no realistic path to them becoming imperialist in the short to medium term. Refusing to critically support them because they might as some point in the very distant future or in some unrealistic, contrived scenario become so is liberal idealism.

One thing to note is that the imperialist powers didn't start out either benevolent or workers' states. They were always or always tried to be colonial, capitalist powers. It's unlikely that Chinese communists would manage to dupe over a billion people into supporting it's rise, only to spring imperialism on them later.

Plus, building worker power to the extent that China has, teaching Marxism to the extent that it does, will naturally create a lot of strength in the people to fight the hypothetical change. The chance to change course without massive domestic repercussions is slim. The western trope is that Asian people are subservient, hence easily manipulated by a 'totalitarian', 'authoritarian' party. But that's not the case. It only works because the people there are 1. aware and 2. supportive. I would expect the same kind of resistance to becoming imperialist in China as there is to becoming socialist in the west.

Outside China, multipolarity undermines imperialism, if doesn't necessarily give the power of the US to China. The US maintains it's hegemony through military and international institutions such as the IMF, World Bank, and WTO. We would notice China becoming imperialist long before it was a remote possibility. For a start, the western world would have to become financially and militarily dominated by China.

We can't be sure. We can only observe and analyze. Here's the most important thing to understand:

Imperialism doesn't work as a model for sustainable society. Capitalism doesn't work as model for sustainable society. When the zeitgeist of a community is one of fear and ignorance, these understandings don't matter. They will build whatever it takes to survive.

But China isn't that community. The contemporary Chinese political zeitgeist emerged after Marx's thorough analysis of sustainable society and demonstration of universal benefit of communism as a social arrangement. China is heavily influenced by Marx and Marxism, by diamat and historical materialism. They teach it to their children, they engage with it at all levels of production and social organizing. The Chinese zeitgeist is fully aware that imperialism means death for China, just as much as they are aware that imperialism means death for the North Atlantic.

Is it possible, even given the above, that China will lose its way and build imperialism? Absolutely! So we must remain vigilant. But China also watched it happen in the USSR. And the USSR was unprepared for the dynamics of reaction. China watched this happen and the zeitgeist incorporates the risk of social destruction into it's workings. More than that, not only did the USSR lose itself, the Soviet leaders who caused it did not benefit consistently or durably enough for most leaders in the world to believe that the USSR revisionist model would benefit them personally. They understand that going that route means death and terror.

It's still possible. But it's not going to be a roll of the dice. It's going to come from a material analysis that shows a path to durable victory for the Chinese bourgeoisie. So far, no one really has that analysis. The Chinese bourgeoisie would need to ally with the North Atlantic to pull it off, and the North Atlantic showed everyone what they do to former socialist projects: shock therapy, bombings, assassination, wealth extraction.

I don't see a path to durable victory for the Chinese bourgeoisie. Even if China went full imperilaist capitalist they would still be in mortal competition with the North Atlantic, but they would now also have fatal internal contradictions that every member of society grows up learning about and is socially reinforced and to which there does not appear to be a solution other than death.

So, critical support is what we are called upon to deliver, and hope is what buttresses our optimism, and historical materialism is what undergirds our hope. Remain critical, but remain optimistic.