Not sure if anyone still following this, but the point isn't that the grammar/syntax works, it's that the meaning and spirit of the phrases is retained when swapping "union" for "empire". Swapping in similar-but-different nouns "democracy" or "monarchy" changes the meaning of the phrase and it wouldn't work. Monarchy is obvious. But note that democracy wouldn't work either - because most likely the 1790's understanding of the Constitution was that States could leave the USA by democratic vote, and the Confederacy did precisely that.

So I didn't mean that the syntax/language works - like you said, of course it does. I meant the general spirit and integrity of the phrase and speech aren't changed in the least when swapping union for empire, in a way that swapping in "democracy" or "monarchy" or other similar-but-different noun would not. When Lincoln said "union", he meant "empire" - and again, good on him, I think, the world is a much better place because he kept the American... union... intact.