Responsibility of the architect in case of KfW interest and further matters

Literature recommendations?

Specialist literature would probably be "Construction Costs for New Buildings and Renovations" by Siemon or "Construction Costs" by Schach and Sperling. The former provides a relatively current, good overview of the subject. It also refers to somewhat recent court decisions regarding planning and construction costs.
The former is almost 20 years old and no longer entirely up-to-date in terms of standards. However, it is still useful as a reference work and well-suited if you want to dive in deeply.

How could we have demanded a recalculation when you are on the construction site every day but are not informed about it?

This has nothing to do with any kind of information, but simply that certain costs remain the same regardless of the quantity. For example, if the excavator costs €680/day and the structural builder distributes this over the quantity, the cost for 34m³ is therefore €20/m³. However, if he excavates 68m³, the excavator only costs €10/m³ proportionally. The same applies to the excavator operator, trips to and from the construction site, etc.

As I said, you must demand this recalculation. According to VOB you have the right to do so. That you have already paid the invoice does not matter; there will be a credit note that will be offset against the next installment invoice.

He cannot provide information on how the quantities came about – he simply would have worked from start to finish.

But he must have told his excavator operator, "Please excavate here to X meters width and Y meters length to Z meters depth." If the operator just digs 2 meters deep instead contrary to the order, because he feels like it, he cannot simply pass the costs on to you. Especially since it will get interesting later when it comes to backfilling; then you will have to pay again

He cannot do much about the prices. The €150 per cubic meter already includes the excavator, etc. and wages. He wouldn't have much profit margin there.

One excavator hour should cost something between €40 and €120 depending on the size. Our landscaping gardener charged €35 net for a 6-ton excavator and the operator probably doesn’t have an hourly rate beyond €70 either. You found out what the container costs yourself (and he probably gets better prices than you do), and he has of course also been richly compensated additionally for the site setup (fence and portable toilet are even extra!). Now you can calculate for yourself what to make of that statement.

Edit: I am not a lawyer, but I would say that by not informing the architect/site supervisor about the additional quantities incurred, he has not fulfilled his obligation to inform. You are his contracting party and the information must go to you!

Direct hit You and I can somewhat assess such calculations. The OP is just learning that, unfortunately the hard way. I, by the way, also learned it similarly, but the prices were not that outrageous... I would also show this invoice to the architect and ask if he would be willing to share in the costs. The OP is paying him for exactly recognizing such things and handling them in the interest of the builder. We are not with Mister GU - "widewidewitt I set the prices as I want"

As for "literature," I am by the way with : Professional literature is well suited if you have a specific problem (for example, I treated myself to the RAL guide for window installation), but to get started as a newbie, that is in my opinion way too much input, which you can't do anything with anyway without the slightest background knowledge. Building is exhausting, stay brave!

What just came to mind: There is also the Bauherren-Schutzbund (just google it). Maybe they can help.