[FredZarguna]

It would be up to each state’s legislature and/or Supreme Court to decide whether the laws enacted by the legislature had in fact resulted in the appointment of any Electors, and who those Electors actually were, if any.

The stricken portion of your statement is incorrect. Without it your statement is correct.

The Florida Supreme Court attempted to arrogate this very power to itself in the 2000 election. The US Supreme Court struck down their claim writing that the plenary power of legislatures to appoint electors is a matter for the political branches entirely, and NO State Supreme Court has any authority over their decision, even the state's own Constitution notwithstanding.

[sourcery]

Who decides whether or not a state’s legislature has actually appointed any Electors? And who decides who—if anyone—has actually been so appointed?

Constitutionally, it would appear that Congress has the final say on that. Congress is Constitutionally responsible for counting the votes of the Electors.