Hypothetical, if the House cannot decide upon a Speaker before 12 Noon Jan. and the Electoral College Vote has not been tallied in Congress and certified, then what?

No Speaker of the House, so next in line is the President Pro Tempore of the Senate = usually the ranking member = will be Senator Charles E. Grassley!

Presidential Succession Act of 1947 (3 U.S. Code Section 19)

In the link here to the Congressional discussions, it was noted that the Constitution has succession rules to follow when the presidency is vacant and there are Cabinet Officers in a President’s administration who can fill in.

In this upcoming Jan. 2025 scenario, there is no Certification of Trump by Jan. 20 Noon and a temporary officer needs to fill in temporarily.