Regarding the falling away of plaster cornice....I did have such incidents due to my cornicing being cracked in places and water/steam getting in behind it and making it unstable. I always knew when this was going to happen because when I taped the cornice with my wooden tool thingy, it had a hollow sound. I managed to save the bits that came away and refixed them. The cracks in the cornice followed cracks running across the ceiling so I also had a good idea as to when this was going to occur....if you follow my drift.