I wonder if the travel agent made a mistake in the pricing they quoted you during Black Friday. I have the same cruise booked, but for July, and I booked it more than a year in andvance, and my price was $4600 for a balcony, no drinks or wifi included. I checked the price during Black Friday to see if it went down but it hadn't, it was around $5900. I know travel agents get good deals but $4010 for concierge class with drinks and wifi sounds too good to be true.
Maybe it was a real price, but I wouldn't be surprised if the travel agent made a mistake when they gave you the original invoice, and that might be why they aren't able to go to celebrity and argue their case now. If they had the paperwork, seems like it would have been an easy fix.
edited to add that I forgot I forgot my price included $800 to make my reservation refundable, so my price would have been $3800 without that. I still think the price you were quoted was really low, but if you have the invoice from the travel agent, they should honor that price for you, regardless of what celebrity is charging them.