If you have evaporation, and add water to refill the pool to the proper level, and the water you add has a lower CH than the water that is already in the pool, will it lower the overall CH of the pool?
No, it will still increase. Evaporation has no affect on the amount of CH in the pool. Think of it in terms of actual weight/volume instead of ppm.
So if you had, say, 1 lb of Calcium in the pool and 2% of the pool evaporated, you would still have 1 lb of Calcium. Even when you add water that is pretty low in Calcium, you are still adding
some to the total so your CH ppm would increase.