The watershed date is Dec. 8th. States must make final decisions in any controversies over the appointment of their electors at least six days before the meeting of the electors on Dec. 14. If a state fails to send its electoral votes to Washington, they are not counted.
The Republicans would pretty much need to prevent electors from voting in ALL of the states they are disputing to have any shot at an electoral win. Of those, they only have state government trifectas in AZ and GA.
Their ability to flip electors or appoint different electors against the will of the people is severely limited. And time is not on their side. That’s not to say none of this possible. It is. But it would require a string of extraordinary events all of which go in their favor.
Here is a detailed timeline of the process from the National Archives: https://www.archives.gov/electoral-college/key-dates