How many states have a way of forcing the electors to vote as they are pledged to? It seems that in recent elections there has always been one or more "faithless electors." It would be more if a Democrat was being told to vote for a Republican.
That's a very good question!
If the compact allows such short notice for withdrawal, it would seem to be OK.
But such issues may be why the Court would ultimately disallow such a withdrawal even under the rules. A campaign is conducted for two years under one set of rules and assumptions, and then in the last few days, the rules change. Big problem.
Also note that even one state pulling out may dissolve the whole compact because the compact would drop below the 270 vote threshold. Such a foreseeable consequence is another reason the Court may simply declare such a compact unConstitutional all the way around.