Continuing to play the Constitution game, I would say the "to whom" is:
1. to the other two branches of government. That would be Congress and the Supreme Court.
2. to the other parties of the Constitution, which would be to the several States, and to the People.
3. One could argue that for 2. above, the Senate acts as agents for the several States, and the Hosue acts as agents for the People.
For "who," I would say the candidates running for the office, and failing to show proof during the candidacy does not relieve the winner, as president, from showing the proof afterwards (amendment XX). Actually being sworn in and taking office does not relieve one of continuing to meet the qualification (and need to show proof), nor does running for a second term. If proof was not provided upon taking office, the fact of having been president should not relieve one of showing proof for taking the office a second time.
-PJ