alternative to packaged unit downstairs
Mini-splits have many advantages, so tossing them out wholesale is a somewhat puzzling response to a house with limited options for running ducts. If you don't like beige plastic boxes (who does, really?) there are several versions that are made to be visually unobtrusive, with the inside unit concealed in a picture frame, or a small inside unit that can be tucked in a ceiling, wall cavity or between the floor joists, either with a faceplate that is the register, or short ducts (a ducted mini-split, as opposed to ductless.)