I believe that this is splitting hairs to a degree that the Constitution can't withstand. Lawyers have turned the clear meaning of that beautiful document to mud for a long time now, because of it's non-legalistic language.
Lawyerese is the gateway to tyranny, where our Constitution is concerned.
It's pretty clear to me, that the Framers clearly intended that if a President was ever found to be unqualified, then the Vice President would act as President until such time as a new President qualified for the office. Very simple.
I would argue that this phrase from Amendment XX applies even after the swearing in of the President.
"...if the President elect shall have failed to qualify, then the Vice President elect shall act as President until a President shall have qualified;..."
I believe that the term President elect was a reference to the personality involved. What else would you call the person involved but what he was? It was a way of identifying certain who should serve. There was only one President elect.
"I would argue that this phrase from Amendment XX applies even after the swearing in of the President."
And I would argue it doesn't. It explicitly states "President elect"....what happens after that should be nothing less than the asshole being led out of the White House in handcuffs by the MP.