OP they have marked your entire deposit as non-refundable. Even if you booked a refundable fare next time the amount from this nonrefundable deposit remains non refundable (minus the $50 pp fee).
You can try to skirt the system, however, they have already covered their bases with that one.
You have 2 years from the date you cancel to use the rest of the deposit. Sorry that the terms were not made clearer to you at the time of booking, but my understanding is there is no getting around it. I had my 2nd pax cancel on me, so I just went solo rather than all the paying $50 pp and all that.
transfer it to a 5 day cruise perhaps that will not cost much more or book a cheap cabin and go and enjoy. The deposit is nonrefundable however and remains that way. Carnival will hold the rest of the deposit until you book another cruise.