What does she think are Nick’s reactions to seeing her going along with it? Why do you think his opinion matters to her?

It is difficult to know what Nick really thinks. By the end of the book he does start having sex with the narrator. In chapter 36, Nick drives the narrator and the commander but he doesn't acknowledge her, even though she's curious about what he's thinking. I think by this point in the novel the narrator has taken greater interest in Nick both sexually and intellectually.