Living room: How to arrange the sofa, TV, and cabinets?

I would widen the west window and omit the south window. Only a little light comes through there anyway, since the roof extends in front of it, and you can't furnish the terrace there.

... and where to put the wall unit? And the south window completely gone ... no, I wouldn’t do that.

I would place the sofa facing southeast, and I would set the window towards the south at railing height like the backrest of the sofa. That is how it currently is in our apartment. We are satisfied.

In the dining room bay window, I would place a sliding door.

... and where does the wall unit go? And south-facing windows completely gone... no, I wouldn't do that.

Wall unit??? I'm out.

Edit: the wall unit can go on the side of the TV.
You can also swap the sides.
However, I would never put a sofa under a window if there is another option.

The change for the staircase (rise 17 cm, tread 29 cm) is in the back of our minds, unfortunately we have not yet received an updated plan.

I quite like your suggestion.

However, we do imagine that light should come in through the living room (south) window. The roof is not lowered here.

Isn't the exterior also ruined if you just paint over the south window?

Regarding the statement "sofa not under the window if possible": I also don't like a sofa with its back to the window, even if the window is floor-to-ceiling, so I like your suggestion. Compared to mine, it also has the advantage that the room remains open, which was my husband's wish. However, I am not sure if we will paint over the south window...