I won't rate this book, at least not yet.
It is a great example of the Brechtian epic theater, and for that I'd have to give it 5 stars.
It is a poor example of a what-if kind of fanfic, and for that I'd have to give it 2 stars.
I decided not to rate it because I understand that, after so many years and such a success of the 7 HP books, you cannot return to your own work as an author. You come back as a fan, half-blinded by the fame and the responsibility, and all you can write is a fanfic.
Although it had too much time-travelling-deus-ex-machina moments for my taste, I'm sure it is astonishing as a play, and I'd love to see it someday.