I don’t think they could do that, either. The count at 8pm on election day wouldn’t reflect ballots that had been received by that point.
Either way, somebody is going to get screwed here.
There is no equal protection under the law if a single state decides election day can be extended for three days - and accept unverifiable garbage as ballots (no postmarks).
If they didn’t segregate the ballots, there’s no recourse that I can see. The PA Supreme Court is criminal for what they’ve done.
The only recourse, if the USSC really wanted to get extreme, would be that they could order an immediate single-day in-person election, with ID verification and with the counting monitored by federal law enforcement.