You are being disingenuous.
SECTION. 3. If, at the time fixed for the beginning of the term of the President, the President elect shall have died, the Vice President elect shall become President. If a President shall not have been chosen before the time fixed for the beginning of his term, or if the President elect shall have failed to qualify, then the Vice President elect shall act as President until a President shall have qualified; and the Congress may by law provide for the case wherein neither a President elect nor a Vice President shall have qualified, declaring who shall then act as President, or the manner in which one who is to act shall be selected, and such person shall act accordingly until a President or Vice President shall have qualified.
The fact that a President dies OR fails to qualify are two totally different situations listed in the language of our constitution. Hence the word OR
It is same uncommon sense that people use to ignore the words natural and or in the phrase "natural born citizen or a citizen..."
Alright then, dying or falling ill and being unable to perform the duties of the President are examples of the President-elect failing to qualify because he/she is unable to perform the duties of the President. Congress is not constitutionally authorized to enlarge or abridge the rights of any U.S. citizen with respect to citizenship status. See SCHNEIDER v. RUSK, 377 U.S. 163 (1964). (”While the rights of citizenship of the native born derive from [section] 1 of the Fourteenth Amendment and the rights of the naturalized citizen derive from satisfying, free of fraud, the requirements set by Congress, the latter, apart from the exception noted, “becomes a member of the society, possessing all the rights of a native citizen, and standing, in the view of the constitution, on the footing of a native. The constitution does not authorize Congress to enlarge or abridge those rights. The simple power of the national Legislature, is to prescribe a uniform rule of naturalization, and the exercise of this power exhausts it, so far as respects the individual.” Osborn v. Bank of United States, 9 Wheat. 738, 827. And see Luria v. United States, 231 U.S. 9, 22 ; United States v. MacIntosh, 283 U.S. 605, 624 ; Knauer v. United States, 328 U.S. 654, 658.”)
Interpreting the fact the Congress qualifies a President-elect as determining the President-elect is a natural born citizen is disingenuous in the light of Schneider v. Rusk. Congress does not have the authority to define a natural born citizen and does not have the authority to determine who is a natural born citizen and who is not a natural born citizen. All we know is that a naturalized citizen is not a natural born citizen.