Regarding the second
I think the Chinese are generally very rational, and won’t do anything stupid. Although if the US looks to be irrational then taking some action to neutralize the threat would be a smart move for them (and possibly everyone).

Also the nature of the changeover (looking at the current situation) would seem to involve the US overspending and thus effectively owing the Chinese a massive debt – the Chinese then get forced to prop up the US economy (and dollar) – like a huge IMF loan with a high likelihood of it never being repaid. Something for which surely they would want some influence in exchange (even if only to put them in a good spot to prevent the US getting ‘depression madness’ like germany did).

But for the most part, I see the influence in part being how china would want international structured to be designed (i.e. their direct influence) but also things like what universities lead the debate on issues, what market is the main market for media etc. Effecting things like whether people are fed a diet of the importance of individualism or collectivism and what sorts of ideas will tend to emerge or be attacked and what sort of things carry even a vague threat of a political or economic sanction.

If you are concerned that some future non-democratic government will have access to past surveillance material I would argue that this is actually the best way to go. If you imagine we lose democratic rights via overt hostilities well no problem, just destroy records before you lose, on the other hand if you imagine a slow erosion of rights it’s much better to have a crowd of dissenters than a few publicly visible people who can be made examples of.

Anyway as far as China becoming a hegemon I doubt they will become one any more than the US is one now. Meaning that the third world countries next door to china will end up needing to do what China says but they will no more be able to tell the US, europeans or other major powers what to do anymore than the US now can tell Russia what to do.

Ultimately it is China’s fast economic growth that convinces me there is little to be concerned about militarily. People who are doing (relatively) well tend to be very opposed to going off to fight and die. There was a reason WWII happened after the economic disaster that was inflicted on Germany after WWI.

On the other hand, I think there remains a danger since I expect China to become hegemon and so the political controls as we know them will be out the window.