Blossom end rot is not a disease, not a fungus, and almost never a shortage of calcium in your soil. It is a water supply problem inside the plant, which changes entirely what you should do about it.
There is no single correct distance between two tomato plants. Here are the three variables that decide it, the spacings most growers work from, and how to adjust them for your own plot.
Late blight is not a fungus, and that distinction explains everything else. Its biology sets the terms: liquid water on the leaves and a moderate temperature. Everything that actually works consists of denying it those terms.