I saw that you had this question on another thread. Quite frankly I cannot find the reference that you made to where someone said that you can see the 10th floor balconies from the 11th floor restaurant.
Anyways, we have been on all 3 Century sister ships including having a 10th floor cabin with a balcony on the Galaxy. We were in Cabin 1060 (starboard, midship). All the balconied cabins on the 10th deck have a roof over the balcony that at least extends out as far as the balcony hand rail. In some cases the roof overhangs past the balcony rail by several feet as ours did. It is difficult if not impossible for anything to be dropped onto a 10th deck balcony from a public deck above.
As for being able to see the 10th deck balconies from the buffet dining room one deck above, I suppose it is possible from certain places of the dining room to see some balconies through a window. This would only be possible by standing by a window in an area of the restaurant where the floor extends out over the sides of the ship. But by and large the balconies are quite private.