Enforced by who? This is actually where Pence’s role comes into play - not in the scenario depicted in your original post - as per the US Constitution Pence would be serving in the Congress as the authority at that moment to choose which electors are the valid ones.
There is a US Code that you are likely referring to, enacted after 1876 that touches on who he should select, but many believe that Code is in violation of he Constitution, and therefore not legal as it is not an actual Constitutional Ammendment.
No. It is enforced by the bodies.