It isn't an all or nothing deal. If the in person voting isn't demonstrated to be tainted, then it can remain. What would get purged would be what is demonstrably poisoned. SCOTUS would more likely take the route that disenfranchises the least amount of voters.
I don't know what PA uses, but it doesn't matter. It's an all or nothing deal. Can't cancel valid votes that were tainted with invalid.
Others are right, legislators likely pick electors. Revote for all other positions later.
It isn't an all or nothing deal. If the in person voting isn't demonstrated to be tainted, then it can remain. What would get purged would be what is demonstrably poisoned. SCOTUS would more likely take the route that disenfranchises the least amount of voters.