Can I prepare Pavlova in advance and put it in the fridge or freezer - would the meringue become soggy? Otherwise, if left in room temperature, how far ahead could I prepare the meringue base?
Hmm, that's a problem, I don't have an airtight container that big. If the meringue is kept in an airtight container, I assume that I don't have to put the container in the fridge? I also have the same question regarding sponge cake - how much in advance and can it be put in the freezer or in the fridge?
You're right. If in an airtight container, the meringue can kept at room temperature.
The sponge cake can be made a day or two in advance - again keep it in airtight container (you may have to go shopping), wrapped in plastic film if necessary to keep in the moisture. Yes, a sponge cake can be frozen, but without the icing/filling/fondant etc.