1. Yes, it can go both ways. However, it is less common for energy to go to mass, for the reason in #2.
2. The C represents the speed of light. You multiply by c2 because a small amount of mass will convert into a tremendous amount of energy. The opposite is also true.
3. Umm, anything? The equation is universal, applying to any instance where mass is converted into energy.
I don't have time to go into detail, so I hope my 30-second-response answers your questions.