Queen Nancy certified him.
No, he was certified by the special Joint Session of Congress called for that purpose, with all 435 Members of Congress and all 100 Senators present. Richard B. Cheney of Wyoming was in the chair.
The rules allowed for adjudication of all challenges signed by one Member of either party, and one Senator from any state.
There were no challenges, and his certificate of election is signed by President of the Senate Cheney.
Now, that's not the only authority with jurisdiction - the voters were first, and the 538 Electors were second.
But that special Joint Session was the last relevant authority to deal with this question, and by their inaction, the matter was closed.