Hi Kirsten, welcome back.

About Luna's problem:
it will not help much but you know that in Germany landlords are not allowed to have general access to rented apartments. Legally they have to give you notice. But I understand that it will be difficult to explain it to them. Most likely they cannot understand why a cat should always be indoors.
Next thing: we have stairs within the apartment and Filou sometimes even balances on the railings but he never fell. The other question: Would the landlords allow you to put a net from the ceiling to the bottom where these banisters are? It would still not keep Luna from using the stairs but she couldn't fall.

However I wish you good luck in solving these problems -I am sure other people here will have more ideas.