First, if the republicans win the house, she would not be the speaker, would she?
Second, I can't find that language in the constitution that says this... Maybe you can help...
https://constitution.congress.gov/constitution/
The link above is the entire constitution. I did a search for the number 20. It shows up in one place... The twentieth Amendment.
Yes, it is not certain who will actually be the speaker of the house after Jan. 3, 2021.
The constitution states that in a case where no one has a majority of the electoral votes for president, the house shall chose by ballot. Also providing that it would be 1 vote per state.
So it is far from certain how such would turn out.
She'd be Speaker until the new House is sworn in and elect a new one. No reason to believe that over 400 House races would go anymore smoothly that the Presidential race.