The builder "outsources" fixed-price services to subcontractors

Hello,
Your answer: “If additional costs were to arise, the engineering firm would compensate for them” implies this.

The general contractor does nothing different, just less obvious. If prices rise in the market, it initially comes out of the GC’s profit; if things get really bad, they make a loss on a contract. Maybe they can afford that a few times. But it can’t go on endlessly.

With the small but fine difference that the engineering office is probably not aware that – with this approach – it legally acts as the general contractor; the engineer in the worst case can hardly benefit from this approach.

That "seems" to be what he believes; see above. Before he can even engage in this business model, he has to ensure that his calculations also match reality; that means there are agreements and thus implied negotiation is proven. I would have to – if I had the time – search through my law forum for relevant judgments; I’m sure I would find something.

Legally, I can’t assess that. Probably then his statement makes sense that, besides the craftsman’s warranty, I also have him for the warranty. Apparently, he is aware that he is indeed liable as a general contractor.

So if, in your opinion, he is still fully liable as a general contractor, what is then the disadvantage for the client?
Due to the contract structure, I can approach both the craftsman and the engineer in case of warranty.
In the event of insolvency of a normal GC, it is not so easy to get to the subcontractors.

Dear God – and I am certainly not religious – keep your childlike faith.

That’s not very nice. Without knowledge of the contract details and costs, neither you nor I can judge that.
And I don’t want to deal with the contract any longer since the offer was rejected due to the scope of construction services, associated costs, and the seller’s conduct.

By the way, you are contradicting yourself here. Either I calculate a fixed fee in my prices or a percentage based on the craftsmen’s services. Then the fee is no longer fixed. Both at the same time is not possible, especially if the billing takes place directly between the craftsman and the customer.
That the engineer doesn’t do his work for free, I have stated repeatedly.

Very easy to answer: if the architect offers a fixed price, he acts as the GC and can therefore be held liable. There are by the way quite a few architects who work that way and have arranged themselves with the risk.

What does the contract structure look like in this variant?
1.) Craftsman <-> Architect (GC) <-> Client
or
2.) Craftsman <-> Client <-> Architect (acting as an assistant in the tenders, coordination, etc.)

If the contract structure is as in 2.), then what is the difference from the engineer?

Overall, I don’t see any major advantages of the construct. But in my opinion, it doesn’t have real disadvantages compared to a normal GC either. It certainly carries a few additional pitfalls in the contract here and there. But there are experts who can review such things.