I understand your feelings towards the current Shin sprite but the more I played through the game, the more I actually came to really like the sprite. It might just be a case of getting used to it. But if the case isn't the same for you, then that's just as fair. Different tastes and all that.
As for Kouya, his sprite (along with Torahiko's) were drastically changed in order to fit their backstories. As an example, Torahiko with his original build and him being a swimmer doesn't make much sense. Swimmers usually have leaner body types. For Kouya, I don't quite remember but I think it was related to his lifestyle and eating habits that didn't line up to the build that he had in the original game. This is what I remember from a post that the developers have made in the past, anyways.
(Also I do think I should apologize, I didn't intend to create a wall of text lmao)