For the record, Jenna Ellis, just might be the worst election law attorney in the history of the Republican Party. Then again, she isn't an election law attorney. If she was, then she wouldn't have allowed herself to run into a wall of laches.

So a little bit about Article II, Section 1.2:

It's far more ambiguous than one would think. It's never been tested: it came close to being tested in Bush v. Gore, but the Rehnquist court never ruled on it because (A) it refused to extend the Safe Harbor deadline and (B) Gore decided to end his legal challenges on the basis that he did not think he could get the Florida Supreme Court to buy an argument that would have allowed for an injunction against the results being certified.

There hasn't been a SCOTUS case that endorsed Ellis' view; but there are a few points of complication:

(A) The Electoral Count Act gives precedence to electors appointed by Governors by the Safe Harbor deadline.

(B) A legislature-appointed slate of electors would not apply by the Safe Harbor deadline because there isn't any state law that allows for the unilateral appointment of electors. Even then, it's debatable that state legislature appointed electors would even qualify under the Safe Harbor provision because there's already election results certified by the Governor. Even if a court bought the Article II argument put forth by Ellis, Congress can simply reject the electors as rogue.

(C) The clause essentially notes that State Legislatures can make election law and appoint electors on the basis of the laws of their state—"in such Manner as the Legislature thereof may direct". The state legislature directed the appointment of electors based upon the election law(s) that they passed.

(D) It's hard to say that the Supreme Court would buy an argument that state legislatures would be allowed to violate their own laws to appoint electors. A court has yet to find that state election officials (or in the Commonwealth of Pennsylvania, the state legislature themselves) to have violated their own laws. Because of laches, a court isn't going to weigh on this in a manner that's going to have any bearing on the 2020 election.

(E) Ellis' argument, which no court is going to buy—not even the Supreme Court—that federal law recognizes some sort of special unilateral power that is not codified explicitly in the US Constitution or in a state's own respective constitution that a state can essentially violate their own separation of powers. Essentially, she's claiming that state legislatures can violate their own constitution because of a dubious legal theory that no court is going to test this cycle.

Pence would not appointed acting President on December 12th, not sure where you got that there. Pence can only be President on December 12th if Donald Trump resigned from office. Regardless, the term still ends on January 20, 2021.