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.
Impossible to you maybe,
If I am China all I have to do is request every Chinese Exchange report their BTC Address & every Chinese person ,
these addresses are then placed in a database that is checked whenever a transaction is attempted, if your address is in the list it is approved because it is in the approved list, if it is not in the list , then it can be denied .
They could even charge a monthly fee to be in the approved list.
If you know anything about how computers work , you know this is not a complicated task whatsoever.
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?
Unless they proved they were in China or Paid to keep the address in the Approved List , it's transactions would be Blocked.
All New Addresses would have to be registered before being allowed to receive any BTC. Again Very Simple. 
It is funny how such simple filters strike terror in your Hearts,
If any of you really trust China as much as you claim, don't worry about it.

But for those of you that can actually think and see the Danger, Plan & Invest Accordingly.

FYI:
51% Security Flaw is Real , Research it or ignore it , your Choice.

Unlike others in these Forums Satoshi did not Lie.