Unfortunately I have not kept mollies long term in saltwater my display tank has always had a problem that has forced my hand into returning them into freshwater, but now that I have bred them and have a steady population I will now begin the experiment of seeing the long term effects on these fish.
My understanding of most brakish fish is that MOST of them only go into full salinity when they want to breed, and then return to brakish water and some even to fresh. With a few exceptions that stay in full salt.