IMO, states choosing to enter an agreement like this of their own free will, is the appropriate way to get a popular vote, if that's what the people want, while still retaining the right of the electoral college.

I personally disagree with a state choosing to sacrifice its voice in such a way, but it's its right to do so, to award its votes how it sees fit.

I think this agreement won't work. When it matters, it would be ignored by some state and any enforcement can be challenged as circumventing the constitution

Yeah, I can see there being difficulty. It's the type of thing that sounds good to a lot of people before they find themselves in a situation where it actually makes a difference. When it suddenly kicks in and people say, "wait, what?", I can see there being legal action or something. But I think it would also be too late, if it's already been accepted as law. It might prompt people to want to start undoing it, but there may be an instance of it applying first.