Don't know how you would feel about this idea or if it is even viable, but decorative curtains/drapes can span quite a distance (doesn't have to be heavy material) across some of your areas. It would allow for easy access and potential for some type of space heater. It would require the thermostat to be in the area most frequently used and for the space heater to truly work. I have been in houses where this type of setup is used. The curtains can be fully retracted for events/parties, etc. What type of windows do you have?