“cannot be required to follow the results of the popular vote in their states.”
Where does the Constitution say anything about popular voting for President? That’s a custom, not a requirement.
Once 535 Electors are appointed by their States, they are free to act as they think best. I’m not sure about the 3 Electors Congress awarded itself in 1960, since they are not appointed by a State Legislature.
Anyone remember Pledged and Unpledged slates of electors? Alabama had such an issue on their ballot in the sixties. Voters would not vote for a Republican, but didn’t want the liberal democrat. They voted for unpledged electors that were free to vote for the Republican. Someone clear me up on this. This may have happened under Kennedy and Nixon.