Good point. It’ll make for an interesting SCOTUS case. But to get there, you need an aggrieved party with standing which means an election would have occurred where this caused an untoward outcome.
I’m not sure how you can head this off before that.
I would pursue the argument that compact is unconstitutional because it denies the non-compacting states' equal protection that each state determines its electors independently from all the rest.
States cannot form compacts to gang up on other states. That's what the Confederacy was.
That's what the Compact of 270 will be.
-PJ