Thanks for being generous about my rambling, Glort. I'd look into the wall and ceiling insulation situation, as well as duct insulation. 12" fiberglass is the standard ceiling insulation here, but if walls can be raised to R19 then I'd want more than that in the ceiling. Your wall that gets sun and warms up in winter; yes, but loss all night as well...the same problem as glass. The only wall heating you really want is a Trombe wall with some sort of insulation outside at night. Perhaps that sunny wall could be used for a hot air panel- which could be covered in summer?
I'm really spoiled in my new house- with double framed R40 walls, the west wall doesn't heat up inside at all in the summer. For my new neighbor's upgraded and enlarged version of my house, I convinced him to have no windows at all on the west wall. I foolishly put a small one in, thinking better night time air cooling in summer, but have found it best to keep it covered with an outside insulating panel all year round.
If only foil in walls- egads.
Another thought- how about adding a large radiator in the central air handling system...with a hot water circ pump loop from your own DIY veggie oil boiler. The boiler would be quite a project... but with your oil experience...