You can paint the wall but it will break up the flow of the space. You can hang a large mirror over the couch but not a good idea if the couch is facing a TV. You will end up with a reflection of the TV. Also whatever you do hang over your couch should be 2/3 to 3/4 the width of your couch or it will look too small.

Accent walls are a thing of the past and will accentuate your room length. Yes use mirrors whenever possible. They will reflect light from lamps, overhead, outdoors. Most importantly select furnishings for the scale of the room and keep it open and flowing. Hope this helps.

I would think you would have sofa on longest wall so that you are only 10 feet or less while looking at the TV while sitting on the sofa.........? Not sure about what you are asking?

But is sofa floating in living room on 12 foot width? Thanks.