how to prevent verdigiris

Because the one has nothing to do with the other. Just because 1 coin with verdigris on it is stored in a given room, that verdigris does not travel through the air and get on the 2nd coin in the same room. If the 2nd coin develops verdigris it does so on its own, it has nothing to do with the 1st coin.

Nonsense, as I said, it does not jump from one coin to another. The only way that can happen is if the two coins are in direct contact with one another. Or, if the 2 coins share contact with a common surface.

Yes it is better, but that is because the salt air make it easier for verdigris to form on its own.