So sorry that you ended with a mess on your hands. Sadly, this is one issue alot of people don't understand when booking their air transportation through the cruise lines.
They cruise lines do not have the responsibility for your flights operated by the airlines, and do not guarantee that you'll get to the ship on time.
Sometimes the cruise line will assist in alternate flights to get you to the ship's next port of call; usually at an additional cost to you. But, it has been so long since I've used air through the cruise lines, that I can't say if that policy is still 100% accurate still.
I'm not sure what type of insurance you got which would only give you a 75% future cruise credit.
Normally insurance will reimburse the full cost, minus the cost of the insurance.
Unfortunatel circumstances that developed for you. The only way to assure you're going to get to the ship on time is to fly in a day early.