that's kind of crazy, i'd ask her if she wants hardwood floors with soot stains or 2 nice, clean, neatly installed registers. but nonetheless, you really do need to use the air in the house as your return air. you don't have to cut a hole in the floor. if your living room has a bunch of hot air registers, use half of them for cold air returns instead. also, you can run duct up, just between walls, then cut a hole in the drywall and install a vent for cold air return, you only really need one big one of these in the center of the house (living room etc.) trust me when i say that this will really solve your problems. still need to get rid of that chimney cap though.