Don't back out just on what I've said! It's your decision. You'll have to go with your comfort level on your plan.
You have other options to consider. How about a one-way flight to Punta Gorda and then a one-way flight from Fort Lauderdale to Plattsburgh? The thing is you'd have to stay overnight after leaving the ship because US Airways is the carrier and they leave in the early morning - you wouldn't make it if you tried to fly out of there on the morning the cruise ends. I was looking at the fares for both Direct Air and US Airways. Using my option would add around $120 per person, not including the extra night of hotel and meals, plus the rental car might be higher since it won't return to Punta Gorda.
But again, it is most likely you will be fine with your original plan. I'd just have a backup plan if things fell apart. I didn't mean to scare you away from the cruise.