What on earth are you talking about? There’s no requirement of proof; no previous president has been asked to prove anything about their birth, including the one who may have been born in Canada. There’s no support whatsoever for your claims, and by your logic, the office of President has ALWAYS been vacant because no one has ever “proven” their eligibility before.
Well, perhaps you can explain what section three of the Twentieth amendment means with the phrase “or if the President elect shall have failed to qualify” What on earth could this be talking about?