The U.S did not face sanctions only because the acts in Guantanamo were not institutionalized - these were particular cases that have been "corrected" by the higher-ups and the responsible parties faced prosecution. There is no legislature in the U.S that would directly stand againts the international laws I mentioned.International Law does not equal "treaty" though. I'll give you an example - let's say that the U.S all of a sudden decides that from now on, torturing POW's, especially terrorists, is legal. You would be in breach of several International Laws alogside the Human Rights bill, conviniently forgetting about the entire Geneva Convention.The Supreme Court established in Reid vs Covert (1957) that the Constitution is supreme to treaties domestically. So if something in a treaty is ruled unconstitutional, that provision is null and void on US soil. So as far as US law is comcerned domestically, my original version is correct.
Trust me, the repercusions againts the U.S would be more then severe. The fact that you "can" do it doesn't mean that you wouldn't be breaching any laws.
Well there may or may not be repercussions (for instance, your terroist example certainly happened in Guantanamo, with nothing coming of it. The US doesn't acknowledge the ICC, etc ) it doesn't change the fact that's the way it is on domestic soil.. Is this right? Maybe not. Is this reality? Yes.
Of course not, because torture is unconstitutional. That's why they put the terroists in Cuba, so it wouldn't be domestic. The supreme court later ruled that the Geneva conventions did apply. Overall it was a bad example on your part because there was nothing in conflict. With ATCA there is a potential free speech conflict, even though I think that the courts would dismiss that and uphold it.
While international law is a great principle, there isn't much teeth to it against strong countries. China has been violating it for decades and is doing better then ever.