Okay, lots of ground to cover there:
- Carbonate causes alkalinity in water. Calcium, magnesium and other metallic ions are along for the ride when it comes to alkalinity. Carbonate takes up hydrogen ions to form bicarbonate, the only way it's soluble, and that's what causes the alkalinity.
- Phosphate in the grains interact with calcium, magnesium and so forth to reduce the alkalinity of the mash (make it more acidic). You can add calcium and magnesium to reduce the mash's alkalinity, to a point. Here's where the concept of residual alkalinity comes in handy.
- Star-san is phosphoric acid and a surfactant. Mixed with distilled water, it'll last nearly forever. Mixed with hard water, you get calcium phosphate, a mineral known as apatite. It is insoluble in water. When your star-san turns cloudy, it's no longer effective. Toss it and start over.
- Softened water exchanges sodium for calcium. It forms salt water, basically.
Until you know what's in the water, I'd recommend buying reverse osmosis (RO) water and adding salts for brewing. Use distilled water for the Star-San - I mix it up four gallons at a time and keep it in a food-safe bucket, ready to use. As for the softened water, use it for cleaning, watering the grass, etc.