The "sofa-like structure" is referred to as a bench, and is a primary part of the heat storage and distribution. It could be possible to get the surface of a mass bench too hot for comfort in spots, which is why there are guidelines for how much cob thickness there should be over the ducts - more near the combustion core and less at the end of the duct run.
You would not want to use sand in this, because sand has millions of tiny air spaces and slows down heat absorption and transmission. Think of a beach on a sunny afternoon - the top is burning hot, but dig down a few inches and it is cool. Cob, stone, brick, anything noncombustible and dense will work for the thermal mass.
The J-tube combustion core with ducts running through a mass to capture heat is the original RMH. There are a few important alternatives developed since, mainly the batch box which allows faster burning and more concentrated heat delivery to the mass, and the "bell" or stratification chamber, essentially a hollow masonry box which absorbs heat efficiently with very little friction allowing easier draft. Many other details are possible if you want to get into the fine points.
I doubt that it would be worthwhile to try to extract electrical energy from the RMH mass. I expect there are more efficient and effective ways to charge batteries.