Congress has total authority under the Twelfth Amendment (one state, one vote).
Read up on the election of 1876—that is where this was tested in the real world.
The PA folks did what they could with the cards they were dealt.
Yes, you are correct about one state, one vote. If that was the reason for the letter then I would withdraw my comments,
But as it relates to the PA GOP deciding to select GOP electors I don’t believe Congress has a role.