The way we play it is that someone with Impervious cannot use any Mystical Powers nor can any Mystical Powers directly affect them whether it be their mind or for real...the exception being a Mystical Power like Zone of Shadows which we see as an area effect fog or cloud that is real and emanates from the person with the Mystical Power. So Dragon Breath, Dragon Wings, Feet of Lead, Harden, Mask of Imhotep, Mesmerism, Strengthen, True Grit, Water Bullets do not work against (or for) a person with Impervious.
With Iron Will only those Mystical Powers that affect the mind do not work (Dragon Breath, Mask of Imhotep, Mesmerism, Water Bullets) and those persons with Iron Will can have Mystical Powers themselves.