Is there a reason why you cannot move the inverts into the QT, use carbon and a copper sponge to get the copper out of the DT, and then perform hyposalinity? All of your fish have been exposed to ich. If you add copper to the display, not only will your inverts and a good deal of biological filtration die, but the sand and rock is going to absorb the copper. Your readings will be off and the tank will be unfit for future inverts. The copper will leach back out of the sand and rock over time. That is rather sad that a "specialty" marine store would tell you that it is fine to add copper into the display.