Hi all, OP here. Wouldn't consider this a zombie thread myself, as it has all the relevant questions handy. Sorry I didn't come back to this earlier, but got a notification that there were updates.
For me, my new passport arrived in time, HOWEVER, to test things, I gave NCL my old (still in date passport but less than 6 months) and it was accepted without any issue. Now, don't take that as a formal proof of what they always do! But I suspect they can't force that rule on EU citizens, and I can't see why they would insist on it in Europe for US passengers either, but who knows.