Architect commissions surveyor without the homeowner's consent.

Exactly, I was looking for answers like that;) I need to find a mistake, how I can prove misconduct by the surveyor in his approach or work...

What a smart tactic would be depends largely on what the court case is actually about (who is suing whom for what?). With every slice of your statements, I understand less what this is legally about and which procedure is at what stage. One thing is certain: the world is bad, and you are right. That rhymes – and what rhymes is good (says Pumuckl). Here’s to a Dujardin, I’ll tune out now! (Those who want can make sense of it from here).

But you don’t tell a coherent story yourself:

a) At first, you claimed to have ignored the reminder from the architect because you thought it was a mistake. Now, in response to the reminder, you suddenly went to the architect in person and offered a "settlement."

b)

The architect commissioned the surveyor on 9.02.2022 for the survey, received the offer for it on 10.02.2022, and the survey took place immediately on 11.02.2022

I wrote the architect on the same day he sent me the contract, via email, that I have to have the contract checked by legal expenses insurance! That was on 11.02.2022

The architect had already commissioned the surveyor before sending you the contract draft? Is that provable?
Who wants to cheat you would at least be clever enough to keep their own timeline under control!?

c) According to your #30, the extension is supposed to cost €750,000...
You could get a single-family house including land for that.
Something is missing from the story here as well.

You want tips/opinions/experiences from us, but you don’t provide relevant information or contradict yourself.
Who is supposed to be able to understand the situation like that?

Without going into the case itself too much, a well-meaning piece of advice: Being right and wanting to be right are two different things.

From your posts I read that you are basically burning to "really show them all". You feel fooled, whether by banker, architect, surveyor or lawyer – maybe soon also by judge and co. Almost lost trust in humanity, you against everyone.

I hope you find a solution you are satisfied with. I think I would consider how much money I want to pour into the dispute – with lawyer, court, reminder fees and amount in dispute. And then sooner or later pull the emergency brake and pay the tuition fee.

Oral contracts can only be concluded under buyers!

That this - regardless of whether with an architect or not - is not true, you know by now (see also Building Code).
But did you also receive pre-contractual information as well as instruction on the right of withdrawal? If not, you could have withdrawn from the contract within 12 months and 14 days. The lawyer could have told you that too, unless you came to him too late.
Then you would only have been able to dispute the payment for services rendered. Your lawyer can also flood you with judgments on that.

Shortly afterwards, I received an invoice from the architect for €18,000! I thought it was a misunderstanding and did not respond at all.

I was already at the architect's before the court with the offer to reach an agreement!
At first, I thought he had made a mistake with the invoice, but when he then reminded me, I went to him and asked if, under these circumstances, that the extension did not materialize, he would agree to €5,000.

That completely changes the story!!!

After six pages of discussion, the fact comes out that you went to the architect a fourth time and were already looking for a settlement back then. Timelines, facts, and information are unfortunately not really your strength. Sorry.

I would suggest that you sort through and document exactly what communication evidence you have and how the process went, both for yourself and for the judge. Probably best to do this really as a list with dates and a brief description (e.g. 01.01. initial meeting with the architect 2h, etc. etc.). Also gather and organize all documents you have (especially the email correspondence).

It only came out here after pages of discussion that the contract draft, your email that you still wanted to review it, the supposed order to the surveyor, and the supposed execution were all roughly parallel in time. It would be great if one could clearly follow when what happened. It’s all clear to you, but make it comprehensible to others as well.

Legally, I have no idea.