I think you need to still define the dining space from the living room. To have proper support and load bearing specs for that area check with an architect and or builder to see if you can make alterations. If they approve, I would put columns up in the corners to replace the walls and if you have to add some iron beams covered with sheetrock, that is okay. It would create a definition to the area without being to intrusive. Best wishes.

The sun is going to be the easy part. The sound is going to drive you to drink. The hard tiled floor together with the vualted space is going to produce tremendous echo. Only a handful of acoustic firms could help with the sound. These firms would be the ones involved in designing opera theatres. Hanging acoustic panels - expertly placed - will be one of the few solutions for sound. There is an off-chance that the panels will need to be hung exactly where the sun pours in...but that would be a chance you would have to take. Unless you can convince the land lord to change out all tile + stairs with cork flooring or rubber AND dressed the walls in cork or rubber, you are most likely going to have to live with the echo. An acoustic solution (including testing, materials, and install) would run into 5 digits very quickly.

It appears to be a running bond pattern with a rectangular tile - is that correct? What size is the tile? Here is what I think I see. The issue that we are all facing is that ceramic and porcelain tiles are 'baked' and during that process the water dries out of the tile and it very slightly cups each tile. Some tiles are worse than others (happens to expensive and inexpensive tiles) and the issue becomes more noticeable the larger the tile. The tile is no longer flat and the middle protrudes out further than the four corners. The popularity of larger format tiles and the running bond patterns has exacerbated the problem. We have gone so far as to contact the manufacturer when planning with a client on the tile selection and they have told us their tile has no problems - only to get very expensive tile that does have issues. And manufacturers will not take it back and neither will the tile shops - even if you ask before you order the tile and you do so based on their recommendation. All of the manufacturers that I know of now say that if their tile is to be installed in a running bond pattern, this could be an issue and they recommend no more than a 1/3 offset rather than the 1/2 offset you have. This does help. Unfortunately, the tile shops are not passing this information on to their customers. A highly skilled tile setter can mitigate the issue if they know to look for it in advance, but some tile is so cupped that there is nothing that can be done. It doesn't matter how much thinset you put on - the corners will just get pushed down into the thinset and the middle will protrude out. There are many tile setters that still don't quite understand the problem or have not been informed of the 1/3 offset. There are tolerance standards for tile, but I do not know offhand what they are. We learned the hard way and we now go over this with all clients in advance and warn that we need an alternate design for the installation in case the tile cannot be installed as planned in a manner we feel is acceptable quality. I do not know of a way to correct the issue after the fact.

I have been living in Japan for the most of the last ten years. Most houses and apartments have a prefab Japanese bath unit. My current place is one of the few that is not one of these units. Our current house in Japan now has exactly what you are describing. It is a wet room, with a door that closes tightly like a shower door. The bath is one of these 2,000 dollar white soak ones we see in the US these days. The bath plug lets out to the floor and the slope is such that it drains down the wet-room's drain. The real missing piece from the pre-fab Japanese units is not so much the drain but the drying unit. I think people forget that when the whole bathroom is a shower the extra moisture that you get from a whole room basically being wet and moist than the amount of moisture you get from an enclosed shower is actually a lot more moisture. In our case even though most of one wall is an open-able window. It still doesn't dry adequately. Unless you are in a desert climate or have someone full time to dry your bath you might factor the dry function in your design. If you look closely at most of the public baths here, their is a wall that opens up completely so that on the day the place is cleaned it is completely dried as well. If you look too closely at some of the less nice public baths you will see mold that looks ancient and will keep you from wanting to take a bath.