Does not sound right to me. I would be inclined to just cancel the cruise and take the $100 a person hit. (You will have the rest in a FCC). You stated that you did exactly as your TA suggested, I would not want the TA to receive any commission on the deal.
If they will not let you rebook the deal that they initially offered, I would book with another TA or just book direct.
I am a little suspicious that they misquoted you initially.
Good luck.