But what of the case of a son of a father (or mother) who never was a citizen? In that case I think there is not way for the son to make a proof greater than what will always be the allegiance to his father's (or mother's) country.
That is my read of the extended discussion in Blackstone.
And that is the first nuance.
The second is that the Founder's, clearly did not fully accept the Blackstone/English common law take on "natural born", where it can be read as accepting just the place of birth as a determination in perpetuity. That is, just because children were born in the colonies, prior to the Constitution or the Revolution did not make them forever British subjects. Viz the War of 1812.
I would assume ‘natural born’ to be considered a necessary (for purposes of establishing a standard) but not in itself sufficient basis for establishing such an allegiance—but it is of course the one in the Constitution.
[Though we all know immigrants with as great or greater an allegiance to this country as many native (natural born) citizens.]
You're really off-base on this one. The Benedict Arnold example doesn't work in the slightest- that would violate Article 3, Section 3 of the Constitution: "The Congress shall have Power to declare the Punishment of Treason, but no Attainder of Treason shall work Corruption of Blood, or Forfeiture except during the Life of the Person attainted."
But what of the case of a son of a father (or mother) who never was a citizen? In that case I think there is not way for the son to make a proof greater than what will always be the allegiance to his father's (or mother's) country.
Nonsense. A person born in the United States does not have to prove his allegiance in any way. A person who becomes a citizen by naturalization proves their allegiance by swearing an oath at the naturalization ceremony.
Following your logic, we could never try a person for treason if they have one or more non-citizen parents. Note that this category includes potentially millions of "anchor-babies."