I think this case is not so big a deal. Yes, a contrary ruling would have favored Trump, and, yes, it would be the spawn of Bush V. Gore, but it would not be egregious or dangerous.

In Bush v. Gore, the important concurring opinion of Chief Justice Rehnquist et al. said that a state court cannot interpret a state statute in a way "changes" the state legislature's legislative scheme for selecting Presidential electors. The U.S. Constitution only recognizes electors appointed as the state legislature provides, so if, for Federal law purposes, the state court is deemed to be legislating, the state court's action in valid.

Consider some polar cases. First, suppose the legislature says that voting must take place on a Tuesday, and a state court "interprets" that language to mean "Wednesday." Under state law, Wednesday it is. But the electors chosen on a Wednesday will not meet Federal constitutional muster because, as far as the Feds are concerned, the state court changed the law under the guise of interpreting the law, and that is not kosher.

Second, suppose the state legislature decides that electors will be chosen by lottery, in contravention of the state constitution. The state courts can strike down the law as being beyond the power of the legislature, and the Federal government could not overrule that holding. because the court is not changing the law but simply announcing that it is unconstitutional. The state might not then not have an applicable law, but that's the state's problem, not Uncle Sam's.

I believe the PA Supreme Court could have struck down the strict mail-in deadline if it really believed it unconstitutional, and there would be no case. But the PA Court tried to thread the needle by upholding the statute and also, at least arguably, amending it judicially to provide "equitable" relief. That's a fairly rare occurrence, and if it is struck down, it won't really set much of a precedent.

It is sad to see what appears to be a division on the Court along "party" lines, because the issue seems to me facially neutral. It's a close question - whether the Federal courts have the power to distinguish between an interpretation and an amendment. In another universe, Republicans might benefit from a delay.

If were on the Court, I suspect that I would treat the PA court's interpretation of the law to preserve its constitutionality as a legitimate interpretation, but I wouldn't get all bent out of shape by a contrary holding, because the principle that state courts cannot legislate with respect to electors under the guise of interpretation isn't all that troubling.