Some things just are that way.
That's how it is.
but I have a hard time with lighting or similar. No reason to want to change things in the floor plan,
I try, in my eyes and with my means, to optimize the lighting situation. I would rather build a skylight in an interior wall or, as you have seen, an open space, just because of the light...
Chill-out areas are not so "light-important" anyway....
TV and chilling work best by candlelight or floor lamp ;)
How dark do you think the lighting situation will be on the ground floor in the couch area at the back,
One could install daylight lighting for times when “normal” light is needed there.
How dark will the hallway upstairs behind the rooms be,
Well... with an open space not so dark in the middle area. Without an open space, I would actually work with a long shed dormer and skylights above the interior doors. It's a kids' floor anyway, so that shouldn't be negative.
Can this staircase theoretically be extended by one floor to access an attic?
That should actually be planned and the staircase set accordingly. You would also have to set priorities, whether a poor staircase location is worth it if you then don’t expand after all. Many things depend on that.
This way, more light (for example through a double casement window) could be brought into the upper floor.
I thought anyway that it would be open to above :D