It's possible that the NCL person you spoke with was not completely up front with you about who gets the $300. Sometimes onboard credits are listed as "up to $300" depending on the type of stateroom you reserve. If the NCL person said, intentionally or not, that you could get a $300 credit if you book right now, s/he may not have had your specific type of mini-suite in mind, or may not have been up front about the credit available for your category of room.
Did any of the NCL folks you've spoken to since then say that the $300 is for a Villa Suite or anything like that?
Some cruise lines have a "Customer Care" department or similar. You could try calling NCL again, and ask if the NCL agent's conversation with you was recorded.
As Dave points out, using a travel agent is something you should consider in the future. Having an experienced advocate working for you both before and after the cruise is always a good idea.
Sorry about this, as it does sound like a real downer. But try not to let it ruin your anticipation of the cruise. Just remember the great time in store for you both.