Her conclusion is incorrect. She neglected to compute g composed with f. To confirm that two functions are inverses, one must demonstrate that g(f(x)) = x. Since g(f(x)) results in -x, it follows that these functions are not inverses.
It appears you've gathered most of the relevant information, but a critical element is missing. The surface represented is parameterized by... and has a normal vector corresponding to this representation with a specified norm. Thus, the integral of... is...