Hi Rosie
you might be right there. Today I am visiting a guy that brings his bees through the Bavarian winter (which is probably easier as it is colder) on 9kg of feed per colony held on six frames in polysteyerine boxes out in fields. He has >300 colonies.
If you had been adding frames of food instead of placing fondant on top of the frames I would have been less concerned.
A supplier here came up with an interesting alternative -they supply fondant in thick polythene packaging. These fit exactly in a frame and can be held in place with tape. They are hung in frames, the pothythene is punctured so the feed can be placed right next to the winter nest. And no worries about the fondant sucking up moisture and it dripping on the winter nest. As the package has a limited feeding area the bees don't bother trying to store it in frames - so I am told - I have not tried them out myself.
Whatever happens I hope your bees pull through!