I'm very pessimistic this is ever going to work. Even if they get above the threshold with everyone still on board, I'm certain this is going to be challenged in court and overturned. Not that there's anything unconstitutional about it, but the popular vote (generally speaking) benefits the Democrats, so there's no way the current Supreme Court is going to allow it. Which is a very depressing view of the Supreme Court, but that's the world we live in.

States can apportion their electors however they want, I believe, even with a hack court.

An Originalist Supreme Court would think differently because of the compact clause:

Article I, Section 10, Clause 3:

No State shall, without the Consent of Congress, lay any Duty of Tonnage, keep Troops, or Ships of War in time of Peace, enter into any Agreement or Compact with another State, or with a foreign Power, or engage in War, unless actually invaded, or in such imminent Danger as will not admit of delay.

For the most part the Supreme Court has let states create agreements on smaller things that wouldn’t be Congressional issues. This is a Congressional issue because they certify the vote so a politically biased Supreme Court is nearly certain to shut it down. Right now the National Popular Vote Interstate Compact isn’t in effect, so no one has standing to bring a case. But once it is in effect, expect a case to be quickly brought because opponents have had years to plan for this.

> No State shall, without the Consent of Congress...enter into any Agreement or Compact with another State

That seems pretty straightforward. Why would any Supreme Court ignore that?

It's also easy to get around. Eventually there will be a Congress that consents.