water is hard or soft depending upon the item of interest you are paying attention to.
Low Calcium and Magnesium means low general hardness (GH). High carbonate means high carbonate hardness (KH).
Your GH might be a little low. I add a powder called "GH Booster" to my RO water to build GH and add baking soda to build KH. You can get some GH booster at http://www.aquariumfertilizer.com/store/aquaticplantfood.php
pH is of course the how much extra H or OH is in the water, where water is H20, or more clearly in this case HOH. It's lagging indicator based upon what's already dissolved in the water. I don't worry about this number as much as the two types of hardness.
Plants such as vallisneria would love your water since they can utilize dissolved carbonate as a carbon source. Most plants can only utilize dissolved CO2 in the water. Some cryptocorynes don't like high KH at all. Many plants don't care very much. For fussier plants, using an RO system is the usual approach to get exactly the water to target at a specific fussy plant.
It depends.
If you do Estimative Index, you really want to do 50 percent water changes. Whenever I get too busy to do this for my EI tank, it gets really ugly in a hurry.
I have some natural tanks with low fish loads and medium light that I rarely change the water. These do OK. They are not spectacular, but don't crash on me either when am too busy to do maintenance for a week.
There are other ways to raise plants as well. For example I use EI/4 dosing in my medium light tanks and get by with monthly or bi-monthly water changes.
There is a good overview on the different fertilizer methods at http://www.aquaticplantnews.com/ You may find this useful.
Good luck with this.