Honestly, I don’t see how you could stop a state from dropping out of such an arrangement if they so choose. The Constitution is quite clear that states can select members of the Electoral College using any method that they choose. Further, interstate compacts without the permission of Congress are unconstitutional. Therefore, the only way to make the national popular vote laws constitutional would be to characterize them as being implemented solely on a state-by-state basis, voluntarily by individual states. IOW, each state would pass a law stating that their electoral votes would go to the national popular vote winner and that this change would only be implemented upon passage of similar laws in enough states to yield a total of 270 or more electoral votes. There would be no legal basis to compel a state that passed such a law to actually adhere to it.
Yes, I've written about this issue before.
However, the way to make it Constitutional is simply to get Congress to approve it.
That said, states today can implement this on their own without needing a compact. They just have to, on their own without other states, start awarding their Electoral College votes to the winner of the national popular vote right now. They don't have to wait for all the other compacting states to join before doing so, if they are true believers in this as the method of awarding their votes.
Lead by example on this issue. Don't wait for a critical mass to form first.
-PJ