Like I said, I haven't seen enough of Kotkaniemi to weigh in on the debate. But there are a few things I would say about your post. First, there isn't a single team in the NHL whose centers don't have more defensive responsibilities than their wingers do. Hence why guys who played center their whole careers but can't hack it in the NHL (Abby, Helm, Franzen) get moved to the wing and not vice versa. So while Zadina may have improved, he's still asked to do MUCH less than a center. Especially a top six center.
Secondly, this year IS Zadina's first year. He played 9 games last season and looked totally lost. Then he looked totally lost in training camp and as a result didn't make the team. He then got called up because of his offense (to replace Mantha) and showed improvement as the season progressed. But he still only played 28 games. Your post makes it seem like there was some marked difference in his play from one year to the next. That's basically an exaggeration considering he still hasn't even played a half season's worth of NHL hockey.
Again, it's pretty clear Zadina is the better scorer. That should come as no surprise considering we all knew that on draft day. But a better player is still very much in debate I'd think.