I can't speak for anyone except myself, of course,
but if we call something CANON, it must be the original novel only, because it was created by Puso himself. Movies, however good some of them are, cannot be called so because, as you stated correctly above, it was Puso assisting Coppola, not even the reverse! And Coppola felt, for whatever reason, that he could change important things in the canon, and that's why we have so many inconsistensies even in GF2, nothing to be said about GF3.
The most annoing thing in this book is the same thing that Coppola did to some extent in the movies. It's the way they both make characters, created already, change incredibly and act in a way they never could.
If you want them to be different persons, capable of different actions, create such characters and write your own stories about them. But we have enough information about them already to see that some things are absolutely unnatural!
There must be some realism in portraying people. You can't just write what you please about any existing character and get away with it .
And after he said in his interwiew that he wrote the sequel to the book, and the book only, not to movies, it is still more strange to see such things written there that make you doubt, at least, that he read Puso's novel repeatedly and attentively.