Re: Volume of ice
Two comments.
First is that I do not know how much but subserface water on earth is not included in many estimations. It is know to extend miles down into the crust. Considdering this it could in theory include a fourth of the water of the Earth. That would make the water estimates for Mars the same as the ground water of Earth.
Second Mars may has started out with the same ratio of water as earth but lost much of it before it had a chance to condence. The time where it was the right temperture range for liquid water may have been very brief. On a geologic time scale it went straight from foggy to frosty.