Since everyone who was a citizen at the time of adoption is dead we can remove the grandfather clause wording. We are left with “No Person except a natural born Citizen [...] shall be eligible to the Office of President;”
Why does the Constitution speak of “citizens” and separately of “natural born citizens”? Why is the word “natural” inserted? It is a matter of allegiance.
A person can be a “citizen” if they were citizens or subjects in some other country first but have come here and met the naturalization requirements. Also, if one is the offspring of a citizen and a non-citizen, then one is a US citizen. However, in both these cases it can be argued that the person might choose allegiance to their former country or to the country of the foreign-born parent or at least the allegiance might be considered divided. That is, there is no natural allegiance of the offspring to one or the other parent’s country. It is this divided or alienated allegiance that the Constitutional provision is designed to prohibit.
If, however, both of one’s parents are themselves US citizens at the time of one's birth, then one is a “citizen” as well as a “natural born citizen”. The “natural born citizen” is one who at birth has no natural allegiance to any other country and the Framers felt could be trusted to be loyal to the US and not act as a foreign agent. In short, a natural born citizen is one who cannot be argued to be anything but; there is no possible argument that he might be a citizen elsewhere. [footnote: Also, in their time, the rules of royal succession held sway throught much of the world and the Founders wished to forstall any potential claims by the crowned heads of Europe or their scions to sovereignty in the US.]
Note that native born is not the same as natural born. Native born simply refers to the place of one’s birth, i.e., of one’s nativity. The term does not speak to the legal circumstances of a birth, merely to its location.
At the time that those words were written. The process of ‘naturalization’ had not yet been constructed.
Of course you know that, but I do not think that you mentioned it.
Your last sentence clarifies it all regarding Kamala Harris.