You get a free 7 day cruise, but picked a 16 day cruise in Europe correct? Did Carnival agree to pro-rate the amount to a different length cruise? I know there are always terms with these discounts, like I had one from something else and it could not be applied to alaska for sure, not sure about europe.
The amount of credit does seem low. I dont know that you can fight it if you were listed as the 3rd pax but think you should get credit as the 2nd pax only because you paid for others. Did you consider just doing another 7 day. I hope you didnt commit to Early Saver rate on the 16 day europe cruise without first checking with Carnival that the discount would be pro-rated how you wanted.
Hopefully, there can be a extension for those who are now pregnant. Maybe you can clear up some of the discussion prior to booking the europe cruise and we can maybe say better if its fair or not.