Execution planning first agreed upon - then excluded?

There is another sentence in the contract: "This purchase contract is concluded upon confirmation of the contract by the contractor." The contract was signed by the seller together with us.....

1. Invitatio (invitation to submit an offer, as the seller (see your note) was not allowed to conclude the contract himself.
2. You agree and have communicated this consent via email.
3. Seller sends the draft contract in advance by email.
4. Discusses the contents with you personally and clarifies further questions etc.
5. You all sign.
6. Preparatory documents are sent.
7. In the meantime, you have submitted the construction project to the building authority (before acceptance of your offer by the contractor) and they have confirmed receipt.
8. The contractor modifies your offer and supplements it. This then becomes an offer from him to you again (§ 150 para. 2 Building Code). You could now accept or reject it or change it again and submit another offer.
10. (actually 9.) He explains his reasons once again (actually nicely) and makes it clear that he does not want the contract if he is supposed to create a "real" execution planning.

"Where would we end up ..." You do not have to like the legal provisions. But here they work quite well. You have understood the term "execution planning" differently, so you were not in agreement at all, even if it looks like it in the text. Fortunately, this is noticed beforehand and both sides can reconsider whether they still want to proceed with the contract as is.

I think it is all quite fine, even if the result may not meet your expectations. The months-long negotiations ended, it seems, in a state where all details are included, just not the execution planning as you imagined it.

And again: no contract has been concluded so far. You do not have a final agreement yet. If you now would not accept, then you have negotiated for months in vain, but so has the contractor. That is fair after all, isn’t it?

5. You all sign.

It's not just a piece of toilet paper being signed here but the construction contract according to the OP. These are two written, documented, matching declarations of intent to carry out this project under the stated conditions.

As a layperson, the first thing I ask myself is:

- why does the contractor have the unilateral right to annul or change this contract through another order confirmation, even after both parties have signed it? Here, the buyer (private individual) is (unjustly?) disadvantaged.

No one is disadvantaged here. The seller probably signed, but it was very clearly stipulated in writing that this contract still had to be accepted by the company itself. If one really reads everything and wants to understand it as it is written, it becomes clear that the contract only becomes effective once the company confirms it as such. However, it did not do that but inserted a change that makes the contract appear acceptable (at least for the entrepreneur). This is probably intended to catch overly generous concessions from the sellers, who sometimes talk faster than they are able to think. In addition, there may sometimes be a tactic behind the seller’s part to first get the signature and then have the entrepreneur cash in on a few minor details. The future builder is then already so pleased (as here) that he will often stick to the amended contract anyway.

Whether one likes the result or not is probably a matter of taste. I find it quite appropriate here, even though the OP probably wanted a different outcome. One must not close one’s eyes to unpleasant clauses ([Acceptance by entrepreneur required, only THEN a contract]). It was clearly, distinctly, and unequivocally regulated, although the OP did not expect the change and was not aware of the situation of the contract not coming into existence. However, this does not change the legal assessment.

It's not a piece of toilet paper being signed here but the construction contract according to the OP.

No, not toilet paper, since this often consists of a few more layers and should be overall softer. But it really isn't much more until a contract has emerged from it. And contracts are formed in Germany through offer and acceptance.

And the contract was also not "unilaterally revoked," but had not even come into existence yet. There was no contract yet, just a few sheets (single-ply, rather hard) of paper. The seller's signature is show, thus worthless. It's even stated in the contract, because the acceptance cannot be declared by him. So what does he declare here? Only that you agree about the content and now the agreement with the contractor must also follow.

Law vs. gut feeling. Usually the law wins.

Thank you very much for your detailed explanations. Everything very detailed and understandable. The construction company has, in our eyes, been impressive from the start. From the manner of communication, competence, and responsiveness – everything professional. Our gut feeling still says that we want to build with this company. Also, in conversations with former clients, we have now been able to confirm that execution plans are created. (Without them, you cannot build a house). However, none of these clients could tell me what scale these were created in. I am still waiting for the expert's response regarding the scale he needs to carry out proper construction supervision/inspection. Has anyone here had experience regarding the scale of execution planning / shop drawings?