Stairs and ceiling - plastering or filling?

Good evening everyone,

my girlfriend and I bought an old building house about 10 months ago - and we had the staircase done by her father.

It was a matter of the heart for him, unfortunately he has had two strokes and is not in good health.
Now I would like to bring the staircase issue to a conclusion.

I know that the installation of the risers went a bit wrong, as well as the construction of the concrete staircase back in 1997.
But we will live with it for now.

The risers are now a bit inside the plaster. I am now looking for a suitable (plaster/ filler) that can be used.

As well as the ceiling - specifically the transition from the ceiling to the base staircase to the attic.

I took photos so that it is easier to understand.

phew. You could run a decorative profile (baseboards) along the contour and basically "overbuild" the step. Then plaster onto the profile. Otherwise, I would mark the plaster thickness + addition on the steps with a pencil, remove the steps/mark the underside and have them sanded in a carpentry workshop. When the steps are gone, you can also plaster better and you don't have to tape off the steps. This way, you won't have the same problem again later and regarding sound transmission, it could also help.

On the ceiling, I would generously cut out the area at a right angle with a utility knife, remove the plaster and apply it with a flat trowel. Then level it to zero with a fine spatula, starting from the adjacent surface over the new surface to the [Einschubtreppe]. You would probably need to repaint the area afterwards or you can put renovation fleece over it and paint it completely again. I would then start completely from the wall/corner. With the fleece, you wouldn't have to cut out an area beforehand.

ps: wishing him a speedy recovery!