Well that salt corns thing is new. Never heard that and I actually doubt the explanation. First, I thought the corns were peppercorns from which black pepper comes from and when you open a corned beef there are lots of peppercorns in there. Not that that means anything. The other possible explanation is that it was originally from Cornwall like Cornish game hens.
It originally meant any small grain seed. The salt used in curing meat was not fine ground like you would get at the table but it was not in large chunks either. It was corned which means it was broken down in to roughly uniform pieces the size of a small grain seed.
This was the size of the salt used for preserving meats so corned (salted) beef as opposed to fresh or smoked.
And Cornish Game Hens are just young offspring of the Indian Game Chicken. Not from Cornwall I am afraid.