Good questions, here are the answers :
1. This is an Architectural Rendering Challenge first and foremost. No problem with sci-fi concepts... we will still have to build stuff in the future. The main focus of this challenge is the visualization, it will be reviewed according to all common technical aspects of making the image as well as original approach, design, the feeling the image evokes in the viewer, etc...
3. Design has less weight in the final selection, though there is an open category for best design so this is something to aim for too :-)
4. It's doesn't have to be able to be built according to current available technology, but you do need to provide a reasonable solution for something that seems not so possible. The story needs to be credible. But again... The visual is what counts most... It's important to allow you to dream a little too :-)
5. Yes,and No :-)
6. No, it can be a mix of 3d & 2d
Hope this helps