Yeah, but what is gained by commitment by 12/13 is "merely" safe harbor. A decision can come later than that, it just loses "safe harbor." 3 USC 5.
Michigan has met this, but if it hadn't, all it means is that a later choice by electors of the state is not conclusive. "Conclusive" means beyond legal challenge and question even by Congress as it counts the electoral ballots.
If Michigan courts found a winner on 12/18, and that process was reasonably orderly, it is a slam dunk that Congress would accept the results even if a challenge was offered in Congress on the day of counting the electoral ballots.
Even 12/19 isn't carved into stone. see 3 USC 2
Whenever any State has held an election for the purpose of choosing electors, and has failed to make a choice on the day prescribed by law, the electors may be appointed on a subsequent day in such a manner as the legislature of such State may direct.
Again, "safe harbor" is probably lost, and the value of "safe harbor" is going to vary depending on the circumstances involved in delay of decision.
......I’m not an attorney, so can you tell me if it helps that the Michigan Supreme Court smacked Jill Stein down today? One would hope so.