Botch on the interior staircase or do I have to accept it like this?

But now, a change of perspective: If you were stuck in the same mess right now, what would be your approach and argumentation to get yourself out of it?

My imagination has exactly at that point a blind spot: imagining that I would act without a plan. I have made a profession out of this inability and now help fellow human beings who have chosen the same position as their core competence. But only under the condition that they participate in the form of expressing their need for advice IN ADVANCE – as I said, simply because I don’t have a time machine.

Yes, I chose two different types of stairs because [...], but we clearly had the premise that the handrail on both should be the same and without posts.

With this decision, you should have noticed that there are many different options requiring detailed planning.

What good is the detailed planning of the stairs to me, do I have a right to it?

If the planning contract included service phase 5, you are entitled to receive it. If not, the contractor can rely on his (improvisation) experience to solve the execution task. In service phase 6, the planner performs the preparation of the tender and in service phase 7, his participation in the award process (and only as GÜ, also representing you in their independent execution). Sampling can only supplement planning, not replace it. If you let the responsible persons start with executions before you have obtained the sampling protocols, you cannot shift the shared blame back to anyone.

Becoming a client and remaining a layperson doesn’t work; or according to Catholic tradition, the immaculate conception is supposed to have happened only once so far and otherwise is theoretical.

However, if there should actually be detailed plans that the stair builder did not adhere to, shouldn't there then be a defect?

The poor workmanship can be fixed - the staircase remains the same...

Do you want this staircase as is or not? Or did you actually approve it this way or not? If not and there is no protocol with your signature under the astonishing color choice - then you did not get what you ordered and I would neither accept nor pay for it.
He can take it back and if he doesn’t want to deliver a replacement, go to a staircase builder yourself...

Be fair - if it was ordered like this - then have it corrected and pay...

GD

What use is the detailed planning of the staircase to me, do I have a right to it? Otherwise, they can just say "It doesn't exist." and that's the end. - Where is the leverage for me here?

As wrote, if there was a plan from the developer and the staircase builder did not adhere to it, then of course that is relevant. Otherwise, if possible, I would also try to consider, besides "who pays for it," what the possibilities are to bring it into a condition that is acceptable for you. Maybe the staircase builder still has some suggestion (or possibly a second opinion with expertise) on how to approach this differently from here. I can't even begin to estimate the amounts involved, but before you end up arguing forever about who is to blame...

Wouldn't it rather apply in this case that the staircase was part of the house via the general contractor and if there is no detailed planning, construction is done as the general contractor always does? I wouldn't have needed to go to the stair builder at all, then I would have gotten the standard staircase in beech parquet glued, with a standard handrail, beech balusters, and finished. I also wouldn't have had to sample a bathroom, then it just gets the standard.

This is how I understood the final payment bond: The general contractor goes to the insurance/bank and says, "The house is finished, but the client is being unreasonable and wants to withhold the entire final payment for minor cosmetic defects (no functional or structural defects). Please pay me, minus a reasonable amount for the cosmetic defects, and make sure you reclaim that from the client."

Otherwise, this bond wouldn't make any sense if the general contractor never gets anything out of it, even if it's just a small scratch in the parquet.

The general contractor goes to the insurance/bank and says, "The house is finished, but the builder is crazy and wants to withhold the entire final payment for minor cosmetic defects (no functional or structural defects). Please pay me and make sure you recover it from the builder."

I am quite sure that the bank/notary will only release it if the builder approves. But yes, withholding an entire installment is probably disproportionate. In case of doubt, only a construction expert can help to assess whether it is really a defect and if so, how much withholding is justified. And if not, I would rather try to find a solution and try to agree whether some compromise can be made regarding the costs...