But once the electoral votes have been counted and certified, and he's been sworn in, he is legally the President. And, BTW, the Framers wisely limited the consequences of impeachment to removal from office, being mindful of when Charles I got the axe and wanting the new country to be more civilized than the old.
Absolutely incorrect.
The eligibility requirements CLEARLY prohibits anyone who fails to meet the requirements from being able to serve as President.
"No person except a natural born Citizen, or a Citizen of the United States, at the time of the Adoption of this Constitution, shall be eligible to the Office of President; neither shall any Person be eligible to that Office who shall not have attained to the Age of thirty-five Years, and been fourteen Years a Resident within the United States."
The Constitution does not allow usurpation to become somehow "legal" just because those who swore an oath to support it failed in their duty. You might try to argue about the definition of terms that are used such as natural born, etc, but "no person" cannot be twisted to mean anything else. No Person.
You stated as well that a natural born citizen is simply someone who aquires their status at birth. What then is a "dual citizen"? The founders specifically placed the natural born citizen requirement for President to guard against the possibility of a dual citizen becoming President. Your "opinion" does not square with this at all.