One thing you have to be careful of is the dates that it covers. If you go with Disney's coverage and you're not booking flights through them, they don't cover anything associated with the flights/transportation or day of (if you are coming in before or leaving after). It just covers the cruise. With insurance purchased independently, you can customize it to cover everything. The trade-off is, of course, is the cancel for any reason (yes, with private insurance, that option is usually quite expensive), which wouldn't be in the independently purchased insurance.
Trip interruption is usually if you have to cut your trip short due to illness or accident or something of that nature, not a missed port or changed itinerary. The only costs you would normally incur with a missed port/changed port are a nonrefundable excursion, if it's the final destination and you were planning to stay there and missed a night at a hotel due to late arrival, or you were diverted to a different port where you then had to pay out of pocket to get to the original port or home. If your policy isn't covering the timeframe after the cruise, then that wouldn't be eligible.
I don't think so. Without reading the entire policy, it's hard to say exactly what it refers to, but I think it refers to transportation to the initial/ending port as it refers to "getting there another way" and "accommodations" and taking alternate transportation of equal or lower class and also references FAA which has nothing to do with the cruise. If you miss a port or are late to a port and have nonrefundable excursion bookings, I think you're on your own with this. Even if it covered cruise ports after departure, you'd certainly be on your own if you're a few hours late due to weather since it says that you have to be unable to get there after trying for 24 hours, so it likely would never kick in.
If you're unsure of any of the insurance coverage, I would definitely call and clarify because there's nothing worse than paying for insurance, having and issue and then realizing that you weren't actually covered.