To the first question, yes you should submit a complete yet preliminary design.  The model does not have to be fully analyzed, but should be comprehensive enough to show your design intent and the structural layout in a computer-generated image.  As for the second question, your final design does not need to adhere to the preliminary design in the proposal, and there is no penalization for a completely changed design.  You should feel free to modify your structure as necessary throughout the next few months, as long as your final poster and presentation accurately reflect your final design.