1. Could the miners potentially make the transactions coming from the US substantially more expensive than all other transactions, or even totally discard them?
Easily thru approved address lists, any address not approved would be charged the higher rate.
That doesn't answer how you're going to target the US alone and not every other person on the planet. And you can't answer that because it isn't possible.
All Exchanges keep track of your Name / Email Address /IP Address , even if you use a fake name and email, using cross reference techniques your location can be determined by your IP alone.
All China has to do is get an approved list of BTC address from the Chinese Exchanges and just block the rest.
EASY.
And for those not using an exchange? You could still easily block half of China doing that. It's only "easy" if you don't stop and think it through to conclusion. Anyone in China could make a new address right now and move some funds to it without using an exchange. How are you adding this new address to the approved list? New addresses are created constantly. How are you going to catch them all whilst also making sure none of them are American?
Further, you seem to address only the points which you find easiest to challenge. What about the case when the majority of Chinese miners decide to retaliate by raising the fees on certain transactions entirely on their own (which I spoke of earlier)? But please don't speak about being rational since in that very case it will be perfectly rational to raise the fees for a certain group of Bitcoin users from an economic point of view...
I did address that:
Then they're raising the fees for everyone, not just the US. There isn't a practical way to single out or target the US alone.
Please elaborate how you plan to identify all these US transactions in real time? I'm not in the US, so please list every single address I've ever been associated with so that I'm not charged more. According to kiklo, it's really easy.