It does not mean that you can decree that something that did happen actually didn't happen after the fact. The entire concept of FSN canon is meaningless if the VN isn't canon.
No, he can. Fate Stay Night is his intellectual property and he can do whatever he wants with it.
Word of God takes priority over fanon, yes. It doesn't take priority over canon. I entirely agree that we should consider what Nasu said, but if what Nasu said is clearly not compatible with canon then canon takes priority, and what Leo is saying is clearly not compatible with canon.
WoG belongs to the canon.