Question

Can the normal be in different plane instead of being in the same plane with the incident ray and reflected ray?

Answer 1

Since we have seen that the incident and reflected rays lie on the same plane that is normal to the reflected surface and the normal is by definition normal to the reflecting surface, it also lies on the same plane as the rays.
It is for convenience that it is assumed to be.Hence we have a common plane for the normal, incident ray and reflected ray.