I have experimented with freezing cake. I find the heavier cakes freeze better than light sponges but it can be done with good results. I usually do all my sponge cakes fresh two days before I need them because they are better fresh but I often do extras and freeze them in case of a last minute job or equipment breakdown. Don’t ever freeze cake with fondant on – fondant doesn’t like the cold! When you crumb coat make sure that the cake is chilled long enough for the icing to harden. If you can do them the day before you are going to decorate them and put them in the fridge overnight that is ideal but a couple of hours is adequate. I have never done a giant cupcake but I imagine the easiest way is to use a giant cupcake mould – there are plenty of recipes on the internet for them. Hope this helps and good luck.