The displacement issue with objects is a big problem for me as well. I spent a long time getting some nice displacement on a light house object I created which I then saved as a tg object into my object library to use in future scenes. The problem is that every time I load the object into a scene I have to go through the whole procedure again to the set the displacement for the object because as I resize the object to fit in the scene the displacement becomes the wrong scale. The only time I wouldn't have to do this would be if the object just happened to be the right size for the scene which never happens.
I would like to request a "scale displacement with parent object" button for objects so that the displacement scales with the object. This way you can drop the object into any scene, scale to size and not worry about the displacement being wrong.
Sorry can't help about the mirror problem.