I think an easy solution would be to add any component (maybe an Arrow component) or a socket that would act as an “Inspect Item Handle” as a child of your first person camera component and move it to the location you want the held object to be away from the camera.
Then when you want to inspect an object, attach it to that component and any rotation applied to the object will be always relative to the camera and that would do what you want.
Then take control of your Turn or Lookup input events if an object is held (simple boolean check or check if the component has any children) and apply relative rotation based on the mouse axis value. Here, i set up something quick for ya, hope it helps.
