Interestingly, Carnival used to do Med seasons for new builds. That ended a while ago. You are correct, part of that reason was build delays. We were extremely disappointing when the very first sailing of the Mardi Gras from Copenhagen was cancelled for this reason. MAJOR Disappointment.
We were on her in late Feb and experienced nothing like what you mention. Special note was our room (Ocean Suite) was 2nd coolest we have experienced. 66.5 at the coldest setting. Sorry your experience was down somewhat.
You pose an interesting question. I have no first hand knowledge of a definition of how long an inaugeral season is, but that said I very secure in telling you you will be fine with your sailing.