Not checkmate by itself, but very scary.
In any case though, if this hypothetical is not hyperbole (and I still suspect it is), it would seem prudent to remedy ASAP.
Here’s the relevant process per the constitution: https://en.m.wikipedia.org/wiki/Contingent_election
Basically, if there is no absolute majority of electors, each state gets one vote in the house for president and two votes in the senate for VP.
All of that only comes into play if there is no majority winner though, so I don’t think just keeping them from voting would do the trick.
The Presidential Succession Act only covers succession in case of removal from office and only until an election can be held, so it is technically true that Pence would be in line, but that assumes Trump was removed and the electoral votes had still not been counted, a highly unlikely combination. Even then just counting the votes would resolve this problem. https://en.m.wikipedia.org/wiki/Presidential_Succession_Act