You can ask the TA to see if Carnival will put you on a wait list for a balcony cabin, it would be a GTY cabin. That is if the wait list is not so long that Carnival is not excepting.
The other item is to be willing to put up the money now for the difference between an inside and a balcony. This would move you way up the wait list. If the balcony is not available the money is refunded, but check to make sure it is not given back in ship board credit.
I know the ship is way oversold, but at this point anything is worth a try.
We have on past cruises requested to be on the wait list and did get the cabin choice, as cancellations came in. All hope is not lost. A good TA should be able to answer your questions or get those answers from the cruise line.