You can get a $200 discount coupon on many cruise lines in "The Entertainment Book."
This includes Princess, NCL, and others. Throw or give your $50 coupons away or use them to line the bird cage. That $50 gift with the limitations and restrictions is only given to encourage you to book with N.C.L. sooner rather than later. If they were trying to make ammends, they simply would have given you a couple of checks for $50 each to cover your inconvenience or vouchers that had some longevity for use on the Norway. But that would have cost THEM money and time.
Also you might try calling a supervisor and explaining your plight. If they are smart and really care about their customers they will extend the vouture to meet your needs, not theirs. If they do not extend the expiration date on the certificates, that will be very telling to you about whether or not you wish to cruise with them in the future.
I'd be very interested to see how a supervisor, not a phone clerk responds to your request. I think a lot of other cruise mates would too!