Yeah, a novel is not a novelization. Let's use the right terms.
Sure. But that depends on whether the book in question was written before the movie was made or after. If the writer based himself on a screenplay (like Millard did for GBU) and not on the movie itself, then I think it is correct to speak of a "novel".
So if a novel based on a film is a novelization (and not a novel), what is a film based on a novel, then? Obviously it's not a film...
No, regardless of whether it was based on the film itself or the screenplay, the work that is produced afterwards is generally called a novelization. Films are sometimes based on novels; novelizations are based on films or film screenplays. If you speak of the "novel" in the case of DYS you will lead the unwitting to believe that Leone's film is an adaptation. This is why Joe had to ask you his question: the formulation you used was confusing.
"the work that is produced afterwards". We actually don't know yet when it was produced. It might have been produced between two versions of the screenplay heavily different (and the blurb on one of the books makes me assume it might be the case) and, as in the case of GBU, displaying elements which change the impression left by the movie:which is not what a novelization is presumed to be, i.e. simply putting images into words. If I give the novelist (or "novelizator", using your vocabulary) words to be put into other words his imagination will have a much freer rein than having to put into words some images. In fact you could consider the screenplay of a not yet made movie simply a stage of the creation of a novel.
A novelization is in the end always also a novel, while a novel is only rarely a novelization.