Buoyant force = m.g
Let V be the volume of the immersed portion of the ice and hence volume of the water displaced.
Then buoyant force = d.V.g (d = density of water)
Hence, m.g = d.V.g
\(\Rightarrow V=\frac {m}{d}\)
Now, if the whole ice gets converted into water then the mass of water obtained would be same as the mass of the ice.
Mass of ice = m = Mass of water
Hence volume of this water = \(\frac {m}{d}\)
In both cases volume of the water is same.
I suppose that the second question refers to throwing stone in river. Well, to see any change in the level of the river one needs to throw the mountain instead..
Ice floats in water. The density of ice is less than that of water. Hence, when water changes to ice the molecules do not come closer. This has to do with the structure of ice. The following picure says it all:
