As far as I know, your plan to get water by chucking ice underground will not work. If you had an aquifer, that work work perfectly for getting water. If you are really fast about it, you might be able to dig down to the caves before your dwarves die of thirst and then they'll all rush down to any water down there to drink.

Otherwise, we'll probably lose 2 dwarves.

As for wood hardness, I beleive that the games tracks hardness for all forms of wood. Whether or not it has proper values entered I do not know.