Sorry to hear about your wife's heart problems, but I'm really happy that her situation isn't as bleak as first thought. That's the most important thing, which I'm sure you're keeping in mind.
It's never a waste of time to try to plead your case, and perhaps you'll get a sympathetic supervisor. But since the whole point of insurance is to protect your investment in case of unforeseen medical emergencies, I wouldn't be too surprised if they don't do as much as you'd like. It's possible they'll transfer some of your money to a future sailing, but it's certainly not something they do often. And the closer you get to sailing, the more money they keep, until the money becomes completely non-refundable. Depending on the ship and sailing, this could be 15 days prior to sailing or more.
Please keep us posted and let us know how your wife is doing, and whether Carnival is willing to do anything.