One of the causes of mutations was radiation, but also FEV floating around on the mainland from West Tek getting blown up. It was a combination of these two that cause the mutations and why the Enclave considers the mainland inhospitable.
Yep yep. But that explains only the core region and maybe DC mutations.
People who did not come into contact with that, would not be mutated.
Also, if SM are sterile, then FEV causes sterility in humans. So, even low doses of FEV should cause some sterility. Maybe not 100% like in SM, but somewhat decent.
Again, if i am in say..Montana, far from core region, I'm pure human. Unless I drink green water by the gallon, then I'm just dead. Or a ghoul.