Stairs again...

Also, I gave this thing a long think (about what does and does not constitute a room and stuff) and I came up with this solution, thanx @eufl for mentioning the platforms! (edit: it didn't work with the people going up and down, so below is the adapted (functional!) version. Both floors are accessible and there are no exploded rooms, but the workaround is extensive...

1) Draw your "cutout area" on the first floor in fence and right click + delete the floor (this area has to be at least 5 blocks wide for it to work!) Afterwards, you can shorten the end with the upper staircase if you so desire.
https://i.imgur.com/wZGVlOb.png
2) Draw a tiny added fence area to the place where you want the stairs to the next level to attach to. Also add 2 platforms, 1 of 1 block next to the fenced area and one of 3 blocks where your downstairs stairs will come up. (There has to be at leats one open space in between, hence 5 blocks minimum) -> You need this 3 block platform for functionality! And if you don't have the open space in between, you can never get rid of the one block platform under the upper staircase :'(
https://i.imgur.com/VeuKqRN.png
3) Remove the fence between the platform and normal floor and lower both platforms to normal floor height, then add both staircases (the 3 block wide platform will disappear due to the staircase placement.
https://i.imgur.com/7gkDk8V.png
4) Remove the fencing around the normal floor (right click, remove fence)
5) Remove the platform by DRAGGING THE OPEN SPACE WALL OVER IT (this step is important to execute like this!)
https://i.imgur.com/UpPHHJT.png
6) I had to click the '3 block platform room' under the lower staircase and, on the floor above it, right click it to 'build ceiling' for the second staircase to be able to open the ceiling up properly. Maybe this will not be the case when a room is present there, but just in case I added the step here.
https://i.imgur.com/CGWXzdk.png
6) Final result: look at your pretty inside room/balcony/double staircase hallway :) This one is fully editable as far as I have tested too. At this point, if you so desire, you can move the side of the upper staircase in 1 block (staircase side first, then fence and place the staircase inwards. It still works and still is a separate room.

After refreshing (going to world overview and back), the platform 'below' the lower staircase will have integrated with the overal floor and no longer be a separate room. This does not influence functionality.

https://i.imgur.com/sQ1uFzv.png

friezes and trims are still a bit finicky though, unless you put the friezes all the way around the building, trims are a bit more buggy...