Hi 1guy, your nutrients will not evaporate from the water which is one of the reasons why you should water with 10-15% runoff to help prevent them from building up in the soil.
As to your soil pH I would lower the water to about 6.0 for now. Your soil most likely has some added lime to offset the low pH of the peat. The lime will slowly get used up and the soil pH will start to drop. You need to watch your soil pH throughout your grow and adjust your water pH accordingly. If you don't have pH Down vinegar will work.
As to cal/mag most autos will need it regardless of type of light but LEDs seem to increase the need, I use it in every watering so yes start now