The GOP legislature will try to seat their own slate of Electors, and the Democrats will go to court to block it until the safe harbor date passes. Maybe Roberts will see the light and rule for the legislature, maybe not.
I think in that scenario it would be the PA state Sup Court issuing a ruling that blocks the PA legislature, and then when the PA legislature appeals to SCOTUS, John Roberts and one other traitor would cross over and issue. 5-4 decision to not intervene.
The way the directors of elections get around this is to destroy the chain of custody before the court can rule, either comingling the ballots with legitimate ones, or throwing out the envelopes that contain the signatures.
The options left to the legislatures are to either accept it (roll over one more time), or assert their power and declare the result tainted beyond repair and appoint the electors directly. This would be an extreme last-resort action, and not one that I would recommend when the process plays out normally.
When faced with the prospect of a legislature-directed slated of Electors or no Electors at all, on what side of that argument would SCOTUS land?
If my scorched earth scenario plays out, the better result for Democrats is for SCOTUS to deny the legislature and keep Pennsylvania's chaos from certifying in time for safe harbor. Then, the election either falls to the House to decide or the Democrats "find" a faithless elector to switch to Biden.
-PJ