Apart from the chaos, what is so terrible about the staircase? An 18 cm rise is very comfortable and a 26 cm tread will be a comfortable 28 cm with an overhang. However, I would like to see how you squeeze a 4 m long staircase into such a dollhouse.
You are also thinking early on that it won’t fit. Removing a concrete staircase will be fun. Then the ceiling openings need to be redone – so how far along is the construction exactly? Is there already a reinforced concrete ceiling on top that you now want to cut through?
For this whole mess, you thought about it for 10 years just to end up with a mega botch job, now the staircase is supposed to be different and you are standing with 3 children in front of a building ruin? Sorry, but my sympathy is limited.