In such cases as the popular vote cannot be determined, or should enough members withdraw such that the majority of Electors no longer fall under the compact, the states can just fall back to their previous methods for determining their elector distribution. That's already established in the compact for the latter case, if I recall, though I don't know that they've a specific provision for another state not publishing their popular vote count. But regardless, worst case scenario, it can just default back to how it already is now.
That would cause the compact to be ineffective, certainly, but still not constitutionally unsound or illegal.
But even that isn't really a true limitation. If they wanted to, they could also just decide to only consider the officially published vote counts of all the states that choose to report it to keep any rogue states from holding the compact hostage. Or they could even just only count the votes of those states in the compact if they so collectively chose, to. I doubt they would, but they could. Again, they have unbound latitude here. Hell, if they were so inclined they could collectively decide to elect the president with the first name in alphabetical order. What's to stop them?