I'd build the whole thing back out as one unit with even shelf sizes both sides. 8cm is not wide enough to put anything on anyway. So if you re-build you can dictate the sizes. All the pics above are nice, but they are all of proper sized shelves. In fact, you could always buy 'off the shelf' units and move them along the wall at will. As your gap is only 8cm you could fill the gap behind the units on both the left and right allowing you to put the unit wherever you like.

That stove can be placed against a flat wall with pipe open so as long as your chimney breast can take the stove pipe I personally think it will be fine. I don’t think it will look wrong if the bench is deeper and if your room is normal sized it will probably be better??

I saw on a gardening programme yesterday where someone had a very busy motorway at the end of her garden. The expert, Duirmid Gavin, said to plant a hedge of Laurel. It’s a thick evergreen shrub that gets quite big but will help with the noise. He suggests it is pruned to keep it in check.

Another thought is if you don’t want to permanently extend the wall between the living room and entry, there must be an ‘expandable’ wall system somewhere on the market. Something that will allow 2 or so panels which can be either pulled out to form a full hall or compacted to open up to the section to show the stairs.