The set up you describe is what I've been doing for years, and I've had really good over-wintering success with it. I've also noticed much the same thing: The bees hit the fondant cakes, and spend a lot of time up in the top of the hive all winter. Logically, that dead space in the top is the warmest place in the hive, so it makes sense that they'd congregate there. But what you'll also notice is that come next spring, the honey stores have also been heavily used, and that the bees feed on all the frames, including those near the hive walls. I think that's maybe the most important point. All of us have had hives that die out in winter yet have a lot of stored honey left, especially toward the outside of the hive. And it looks like the cluster was unable to get to the honey. I speculate that the warm dead space at the top promotes easier movement to and through the stored honey frames. In fact the UM Bee Lab suggests pulling a frame from overwintering boxes as another way to promote easy movement. I'd also speculate that the fondant cake might not be necessary as much as the dead space. But I've not been willing to gamble hives on that experiment. Regardless, what you're seeing isn't unusual. I just try to use the few nice days in winter to make sure some surplus feed is still available, especially in late Feb. and March. Since others also use "shims" in their feeding approach, I'd like to know if they see the same thing.