From all the posts I have been reading of Hamilton's writings on the Electoral College, I get the impression that the original intent was for the states to merely select electors, and that the electors would elect the president.
Yes, but the important part that you didn’t include when you quoted me is that they give no instructions to the states on HOW to select electors. In fact it specifically says they can do it however they want: Each State shall appoint, in such Manner as the Legislature thereof may direct. So if the Legislature directs to leave it up to the national popular vote they can do that.