They may remain married even if he is married to someone else? You wrote that he paskened that cherem d’Rabeinu Gershom does not apply in such a case.
From the little I saw, it seems so. This obviously would not be practical in a country which prohibits polygamy.
So in a case where an eino m’daber v’aino shomeia is the only brother, the almana remains an agunah for the rest of her life?
I believe so (obviously, you’re assuming that the brother outlives her).