How many countries have a "natural born citizen" requirement for their head of state/head of gov't?

Do you have any details?

It would be rather odd, since life peerages are entirely a statutory creation - the Lords in their capacity as judges of peerages ruled in the 19th century that there were no life peerages at common law.

They’re talking about changing the law to let the Dark Lord do it, which suggests strongly that he can’t now.

There were a number of medieval examples of peers surrendering peerages. However, in a series of cases in the seventeenth century, culminating in that of Viscount Purbeck in 1678, the House of Lords ruled to the contrary and this was upheld by the courts in 1961 in Tony Benn’s Bristol South-East election case. The 1963 Peerage Act was specifically introduced to change the law on this point.

However, the 1963 Act only applies to those who inherit a peerage. The view, reasonably enough, was that if you have some moral objection to peerages, you have only yourself to blame if you accept one. The pre-1963 rule therefore still applies to life peers, as well as to the tiny number of surviving persons who have been granted hereditary peerages.

But this has recently been an issue anyway, with or without Lord Mandelson’s (almost certainly non-existant) leadership ambitions. Baroness Ludford had asked to be allowed to disclaim her life peerage in order to stand for re-election as an MEP earlier this year. She wanted to do so because the EU had introduced a ban on MEPs concurrently sitting in national legislatures. The law was instead changed so that peers are disqualified from sitting in the Lords if they are also an MEP. A similar disqualification had already been introduced in 2005 for the law lords when, later this year, they become the judges of the new Supreme Court.

As the issue has already been aired, it’s therefore not too surprising that there is talk of allowing life peers to disclaim as part of any package of further Lords reform.

Well, probably not then. I was going from memory and I can’t find a cite.

And what clause in the unamended Constitution imposed an ethnic test for high office? The 3/5s rule applied to the census.

The citizenship requirement. African-Americans were not, as a rule, granted full citizenship until the passage of the 14th Amendment, except in a few Northern states.

1.) That is not an ethnic test at the federal level, but rather at the state level.
2.) As you say, states differed.

In other words, Dred Scott relied on bad history.

Whether or not the Court’s history was bad, its ruling was not just about citizenship at the state level - it held that slaves and their descendants could not be citizens of the United States, as summarised by the wiki article:

Since the President has to be a citizen, the Court’s holding meant that black persons were constitutionally barred from being President.

Yes, but:
1.) The Court is not the Constitution.
2.) The Court ruled in 1857.
3.) The Court was pretty clearly wrong on the merits. It relied on the argument that African Americans were not citizens of the states at the time the Constitution was ratified. But see this point from the dissent (taken from the wiki):
“Nor, these justices [ed. the dissenters] argued, was there any Constitutional basis for the claim that African-Americans could not be citizens. At the time of the ratification of the Constitution, black men could vote in ten of the thirteen states. This made them citizens not only of their states but of the United States. By the time of the Dred Scott ruling, however, five of the ten States that allowed black men to vote had either restricted this right in some way or completely withheld it.”

Curtis’s dissent is here, but it’s rather hefty reading