Trip insurance is a personal call. For our family, the peace of mind is worth the cost. Trip insurance will cover you for the pro-rated cost of your trip if you have to miss any of it (you get sick, you miss your flight the day of the cruise, etc.) It also will cover any medical expenses (including medivac) if needed while on the trip.
Frankly, for a 3 day cruise, the cost of the insurance compared to the cost of the trip makes it not look like a good deal. Unless I was worried about a medical problem on the trip, I might not buy it.
Transfers are another personal call. For our family, we could do one-way rentals much cheaper than transfers. And we were able to come and go on our own schedule, stop as many times as we wanted, etc. Of course, that meant we were free to get lost too! The Disney transfers typically arrive at the port a little later, so if you're set on a particular Palo reservation, it might be gone by the time you're able to check in. Budget, Hertz and
Avis all offer one-way rentals to the Port Canaveral area. Also, for a 3 day cruise, it might be worth getting a week long rental at a great rate, and parking your car at the port (there is a fee, maybe $10 a day?).
Good luck, and happy cruising!