I agree with every of the point you have here, when we talk of them controlling bitcoin, I am sure we would not be looking at the aspect of them being able to control the supply but the value is what they can control, and they may actually have a hand in the ways and manners for which the value of bitcoin fluctuates, but I believe that somehow, there will be a day that even china being the largest and richest country will not be able to manipulate the coin and this is where Satoshi becomes more intelligent than them.
Hope you are aware that the country is not behind mining of bitcoin and individual who invested are running the mining as a huge industry as you need to invest millions to run a farm if you are looking to have substantial hash power as it is a competitive field and there are business people who have invested elsewhere and anyone can run a mining farm as all you need is the money to purchase all the equipment and hence you cannot call that some country is controlling something, this is a decentralized market and mining is not a monopoly that is given to anyone.
They are able to manipulate the coin because there are still coins to be mined, and when it gets to that point where they will not have any coin to be manipulated, I am sure that their power will be reduced a little bit in the regulation of this coin.
How you think they will manipulate the coin, by inflating the price of bitcoin and if that is the case it is good for traders, other than that how they are going to manipulate as they are mining the coins to earn the profit and there is no way they will do something to harm the market.