The key constitutional phrase is "The person having the greatest number of votes for President, shall be the President, if such number be a majority of the whole number of Electors appointed;"
If, in some alternative universe, the Trump supporters managed to reject the Pennsylvania votes, would they try to claim that the electors from Pennsylvania had been "appointed", but their votes were rejected?
Of course it's an absurd claim, but the whole situation is absurd. Basically, if they throw out the law in the first step, how will the rest of the situation play out? You can't throw away the law at the beginning and then claim to be following it at the end. At that point, it's up to the courts and, ultimately, the guns. Fortunately, there's no way that they are going to have the votes to disallow even one electoral vote, but it's sad that the subject is even being discussed in a serious fashion.
There is one takeaway that I will repeat for those confused about the law, and who read things in various places. I have read numerous articles and comments recently that basically assert that if anything goes odd along the way, the House of Representatives picks the President. The important thing to understand is that those assertions are not true. The House of Representatives only gets to pick the President after a number of other steps have been taken, including counting the electoral votes. You can't skip all those steps and go directly to the House of Representatives.