The US apparently liked the concept enough to make it into a law: Link
Hardly. Sumter was not the property of the confederacy so they couldn't exercise control over it.
Not according to the Confederate Constitution which was the law in place in the seceded states. US law did not apply, but you know that.
Again, private property. And during war time. We're talking about your claim that eminent domain is used to seize property owned by foreign governments in time of peace. What are some examples of that?
Not according to the Confederate Constitution which was the law in place in the seceded states. US law did not apply, but you know that.
Neither did any confederate law, which didn't apply to the territory of another sovereign nation. But then the rule of law was of little interest and less importance to the Davis regime was it?
Seems that in the USC sections cited in footnotes 2 and 3 at your link, USG claims precisely the powers over alien property -- including that of alien governments -- that South Carolina claimed over the federal installations within its borders in 1860, after separation.
And they did offer compensation to the United States, which Lincoln brushed aside in his "war surge".