I don't think you'll get anywhere when trying to fight this out with the hotel. I assume you booked via the ICH general website? Then you should challenge the the callcenter in the States and involve customer protection groups. A contract cannot simply be amended like this. Let it be the ICH's problem and not yours.

These outrageous Christmas Eve and New Year Eve Dinners are a reason I don't want to stay at hotels during that time. I ran into the same story in Turkey. We paid an all-inclusive package and the hotel just said upon arrival "you have to pay for the Gala Dinner".

HTB.