re: a President cannot add an amendment single-handedly.
It’s called an executive order, national security council memo, national security directive and if it’s a matter of national security, it does not have to be put into the public record. Presidents have been creating secret laws and secret governments outside of constitutional limits since the Civil War.
I know—the Presidents in recent decades have gotten away with lots of things that would have appalled the Founding Fathers. As you indicate, some of this goes back to the days of Abraham Lincoln. But what’s the Constitution between friends, anyway? The Bill of Rights is in many ways a dead letter nowadays, especially for people deemed political opponents of the powers that be.