Daox: You can do fun things with color. Reds absorb the least thermal gain (that's why it's red); greens, blues, grays, and of course black are best. It's a nasty job, but you might consider changing the reddish floor tiles to a more thermal color. You could use the opportunity to lay down a radiant barrier underneath. Likewise the back wall, if the sun gets low in winter.
Cellular blinds have an R value of around R-2 or 3. Not great -- but uninsulated glass is around R-1, so it still helps. We have single-cell blinds on sloped glazing, and it's a terrible thing if we forget to close them at night! Some (expensive) blinds have a silvered lining, which can be hugely effective at stopping re-emission of heat. Coated windows likewise. But for a cheap solution, stitch up some winter drapes: a nice heavy fabric on the inside, a layer of silver bubble foam, and some polyester ticking on the window side. In conjunction with your cellular shades, they will reduce losses by convection and radiation.
Ah, radiation. It moves in straight lines and abhors partitions or hallways. My house is designed spoke-wise in an attempt to get the passive solar into each room, but older homes aren't usually that open. Your big double doors should carry heat into the house fairly well, though transom windows and vents near the floor set up a nice convection loop. You could go for some tromb wall thing, but I'd rather use free-standing solar hot water panels and active solar. Or put some black or blue 55 gallon drums of water in the sunroom in winter.
Nice-looking house!