If Scali doesn't step up then you can camoflauge it with paint. Experiment beforehand with a tube of black, grey and white acrylic paint and a stippling brush. Mix up different blends until you have the right colours and stipple it on. Start with a base coat of grey. Let it dry on an old tile or similar before assessing the colour as correct. The good thing is that you can wipe it straight off with water if you make a mistake or it doesn't match. When we replaced our kitchen there was a patch of raw pine exposed on the floor. I painted it with watered down acrylic paints and painted in some knots with brown paint and no one has ever picked it. Took a few tries to get it perfect. I assure you it won't stand out like it does now. i've even used texta to hide scratches on furniture and polished floors. You might consider that as an option too. Lastly I spilt scented oil on a my dressing table and it created a white film. I put some Brasso on a piece of cloth figuring I had nothing to lose and it's supposed to work on water rings on furniture. It took the white film straight off with no damage. You could try some on a cotton bud to see if it helps. You never know. Good luck.

Look into terrazzo, it's basically polished concrete with style. Terrazzo is a poured coloured concrete with glass or stone chips in it to give it a variety of looks. It's then polished to a high lustre. It comes in any colour like from bright whites to black and you can choose the mix of aggregates to make your own custom flooring. Commonly seen in tiles, it can be poured as well to avoid the tiled look. A similar colouring to sandstone would be easy to achieve this way with a more durable finish. Terrazo's can also use interesting agregates like sea shells and limestone fossils to create a complete custom look. An alternative to a high thermal mass flooring would actually be the installation of phase change material mats in area's subject to direct light. Phase change material is a polymer used in the transport industry that either consumes or gives off heat as it moves from a solid to liquid state. The material is know to radiate either heat or cold back into the environment for many hours in a controlled release after several hours of charging through exposure to heat or cold. It's an inert technology that never wears out and innovative architects are beginning to incorporate it into passive housing design. The panels are manufactured to be quite thin so you could hang a pcm mat on a wall for example and easily cover it with something innocuous but it would passively heat the house.

Is the sitting room at the top of the stairs? If so you could carpet the top riser. See attached photo.

everything you've suggested sounds ideal and you will only need to build a frame underneath the stairs to create a wall to the side of the steps and directly under the top step to allow for a door and cover with the same wall lining and either add an electric or battery operated light under the stairs and shelving to suit what you want to store