The cantilever staircase does not hold, the stair builder is not a staircase builder

Should the exposed concrete remain? Or will it still be plastered?

I would immediately advise the OP to seek support from a legal counsel, anything else will only delay the matter.

That will only delay it even more.

A structural engineer with expertise should preferably calculate that first.

That only delays it further.

I see it differently. Set a legally valid deadline for rectification if not already done, and either the third botch-up happens or nothing at all. Then have a remedial measure carried out and sue the cost difference between the specialist company and the botcher from that very person. It only works afterwards. Waiting until a legal dispute has been conducted was not what I had in mind either. As we could see here from a picture, the good man apparently had no idea, neither about what he was doing nor how he was doing it.