Don't believe you are overthinking this.  You will need to go low poly with the remainder of your populated scene, especially if they bear no relevence to the main characters.  The original Vicky and Michael might fill that need.  Also, textures should be low res to ease the render load.  If you think this might be too intensive, try render passes of back-, mid-, and foreground and combining the images in a 2d program.  Your workflow approach is just as important as your set, and you should explore other options if your original concept doesn't work out.