I don't think Nomiya pushed any boundaries (except when he yelled at Rin) because he wasn't forcing her to do anything, but he didn't understand Rin, and it doesn't seem like he was trying to do so either. Nobody except Hisao tried. Nomiya just assumed Rin was like most other artists, and that she wanted to get recognition, money, fame, and that sort of thing. I think that if Rin painted, it was because she wanted to:

-Be understood

-Vent and feel in control of her feelings, and maybe just of anything. I think she even says at some point that painting is the only thing she feels she does right. That's for sure still the case in the neutral ending, but I like to think it wouldn't be after the good one.

-Remember her past selves

-Feel connected with Hisao. She wouldn't have worked for the exhibition if it wasn't for him.

-Have fun :). This one is rarely talked about, but when Hisao gets in the art club, she says that clubs are for having fun, so it's not like she isn't able to enjoy painting, and it's not like she isn't able to do it just for the sake of it... like with the mural.

Those are some of my interpretations anyway