As you have found out, "Ich" is very contagious and can spread to other fish within a week. Cleaner shrimp are not very effective and my do nothing at all to reduce the number of parasites on the fish. Lowering the specific gravity to 1.019 will not stop the spread of "Ich" but will be dangerous to your invertebrates, especially the sea star.
It is possible for the remaining fish to acquire an immunity to the "Ich" before it kills them. This might save all or at some of the fish. However, it is possible for them to acquire only a partial immunity which means they will still be able to host parasites but not show signs of infection. When you add any new fish, they will become infected.
There is nothing you can do to treat the fish that are currently in the tank that will not harm the invertebrates. The only practical and effective treatments for "Ich" are copper and hyposalinity (but that should not be used together). Neither of these can be used in the presence of invertebrates.
My advice would be to set up a quarantine tank large enough for ALL the fish and move them ALL from the main tank. Treat them with hyposalinity for a total of 4 weeks after the last spots leave the fish. This will result in the main tank being left without fish for around 6 weeks which will kill all the remaining parasites.
I suggest you read
Marine "Ich" and
Hyposalinity. It also wouldn't hurt to read the rest of this thread as many of your questions have already been answered.