1) Concentration Camps, I believe first were established by the British during the Boer War. The Nazis has Concentration Camps for political dissidents in the 1930s. The Nazis set up a small number of Death Camps in the 1940s after the Wannsee Conference.
2) The Weimar Constitution predated the Nazis and was (mostly) not a bad document. I don’t think the Nazis technically cancelled that document,
3) One fault would be Article 48 which allowed the Chancellor to ignore the constitution if a state of emergency was declared. Hitler came to power, declared a state of emergency and basically made himself a dictator. Nothing prevented this.
4/5) The US Constitution, in my opinion, should have established this country as a Christian nation, but with no specific denomination. No established government controlled church. But Christian. Also the Second Amendment should have been written more clearly. I believe it gives Individuals the right to bear arms, and I believe it blocks states from infringing this right. But that interpretation has not been universal over the years, as we well know.
Also, I think Congress should have been mandated to attach the specific Constitutional clause which allows them to pass each and every piece of legislation. Establishing a Department of Education? Where does the Constitution say that the federal government can do that?
Also, I think income tax is evil, and I think the constitution should have explicitly said that it was not allowed.
Three more points — The “general welfare” clause should have been explained. I believe it was intended to guard against government corruption and to ensure fair dealings in government rather than roads and bridges and dams and EPA and DEA and all the rest of that.
The Interstate Commerce Clause should have been explained. I believe it was intended to guard against states engaging in trade wars against other states rather than to allow the Feds to control any and all economic activity that crossed state lines.
If the Founding Fathers had succeeded in agreeing to an end of slavery, we would have been so much better off. If not an immediate ban, then a phase out over 30-50 years. Slavery was wrong. They knew it. But politics and economic concerns got in the way. The Great American Sin, in my opinion.
"A well armed militia, being necessary for the security of a free state"
as opposed to an unfree or shackled state, the "right to keep and bear arms..."
is a method for helping to maintain the security, the preservation, of that state of freedom.