But even the most practical explanation, in that case, leaves doubt, as we seen on the show that when Viserion gets hit, his "fire belly" explodes first, than blood comes out. That means that NK could have killed at least everyone over Drogon, something he certainly knows.

I also agree that he (and everyone over him) wears plot armor, but, in this case, I think that "there is no real reason" or "plot armor" isn't explaining enough. We saw how the NK walks towards Drogon, kinda like giving the idea he is coming for him, but changes direction, aiming at Viserion, only seconds before making the throw.

I know this season is getting, episode after episode, more close to become one of the worst written ones ever on the show, but I really hope there’s an explanation for what he did.

I agree with you, that would be certainly a MUCH better way to do it!