Basically what happened is PA Supreme Court, being a joke it is, allowed dems election rule change by litigation, agreeing to late ballots being accepted without almost any verification. GOP then petitioned the SCOTUS for emergency relief injunction - to build in a protection for futher litigation (ballot separation). This was the time when SCOTUS had 8 justices. They took the case and it was 4-4 split (Thomas, Alito, Gorsuch, Kavanaugh vs lefties), so the petition was rejected.

About that time Barrett was sworn in, so GOP pushed another version of that same request through courts. Election officials seeing writing on the wall KIND OF agreed to the GOP request, and introduced voluntary measures to separate ballots (normal vs late). The case eventually reached SCOTUS again, days after Barrett was sworn in. There was still time to FORCE those rules into law, and not rely on PA election officials good will (or lack thereof).

This time Barrett decided not to take part in the case, citing late arrival. Which is very convenient explanation, but also doesn't hold water - this was simple case of emergency relief, not full actual case with months of litigation. Unfortunately Kavanaugh also joined lefties that time, so the case was 3-5. Thomas, Alito and Gorsuch dissented, stating that this still can be litigated after the elections.

The thing is, it was MUCH, MUCH easier to fix beforehand. Now it's the complete legal FUBAR, it will require HUGE, unprecedented SCOTUS decision to do the right thing here.

Sauce: https://archive.is/eH4Qk