The simplest way is to open the built-in library of D5 Render and place the furniture and objects. Alternatively, you can download and load external models into D5 Render, then apply materials and proceed with setting the effects.
Yes, of course, but what about the right angle, the shadows etc. ? Can we display the photo directly in the viewport background on D5 so that we can rework it like a layer?
Kinda depends if you really want to be sure that the furnitures are ok for the room the original photo is not really relevant. What really relevant is are the dimensions and the materials of the existing room including windows and that can be modeled.