Even if "undoing" a past election would be something allowed by the US Constitution (it is not), the only result of this could be Pelosi taking over until a new election is held. If the election would become null and void it clearly had no winner. And in absence of a President and a Vice-President, the Speaker of the House becomes a temperary President according to the Presidential Succession Act of 1947:
"if, by reason of death, resignation, removal from office, inability, or
failure to qualify, there is neither a President nor Vice President to
discharge the powers and duties of the office of President, then the
Speaker of the House of Representatives shall, upon his resignation as
Speaker and as Representative in Congress, act as President."