You can make it well in advance, especially if you aren't expecting it to be eaten. Just don't put it in a closed container or refrigerator and keep it out of direct sunlight. A cardboard box works well for me to keep dust off but keep the air flowing.
AI have not had that as an issue. I have fondant figures that are two years old and a rice krispy figure covered in in fondant and modeling chocolate that is a year and a half and are fine. I wouldn't encourage anyone to take a bite, but they look as good as the day I made them. As long as they are not exposed to moisture or heat, they last a long time. I make both my own marshmallow fondant and modeling chocolate, so it's not a preservative thing either. Sugar preserves very well.
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