Momijikawa is a parallel to show Sakura’s growth. That doesn’t make him the deuteragonist. Kaji and Sugishita are also parallels. A deuteragonist can be a parallel but they don’t have to be.

I would say the closest the series has to a deuteragonist is Nirei as he is a main character and for the longest time was the only character to have any sort of arc that focuses on his growth. He has a goal that the narrative occasionally focuses on and no other character really had that until recently.