1. I don't really know what an asset's showcase map is but you can use any assets that you have the right to use. You do not need to have made all the assets yourself.
2. By demo game, I assume you mean a short game that you intend to make into a longer game. This is fine. However, if it doesn't come to some sort of satisfying conclusion by the end, you may not get very high marks in the story category.
3. I don't know what you mean by "level." There are no levels in adventure games. Can you explain more what you mean?