When I enter Audience Areas and/or Listeners Seats, are the seats already considered "occupied" for the mapping calculations? If not - how do I simulate real listeners?

Neither Areas nor Seats will have any influence on acoustical calculations. To simulate the absorption of people you should just add some 2fold (coat) faces at the spots of interest, propably right beneath your Areas. For example a new face simulating audience should be a 2fold face coated onto the floor face (replacing its acoustical properties). Don't forget to assign the "Wall Material" of desire. It's just the same with windows. For more details on Coats see below.

Please note: Only Faces and Loudspeakers are considered in all the different types of computation. None of the other.