Incompetent architect - what to do?

...how, with the information regarding liability it is a bit late? Do you always do that beforehand before problems arise?..

Very few builders have the appropriate legal knowledge (contract design, liability, etc.) within the family and are therefore actually dependent on professional help in this regard. At least from the initial situation here, this apparently is not necessary, which is why the whole thing puzzles me a bit.

...pulling the ripcord means exactly what?...

Separation from the previous architect! Building with open planning always means and presupposes a relationship of trust. The same applies, by the way, to my work!
If the relationship of trust is permanently disturbed by significant events, continuing would hardly be sensible for the client.

...What, I have to pay an expert now to supervise my expert? ...

There is no talk of "have to" here, but rather of what makes sense, as your experience clearly shows!

..., since the experts did not find a common denominator....

That such things are not easy and certainly cannot be arranged quickly should be understandable.
On the other hand, there is a common denominator with some tolerance for every task!
In which economic system do you actually live?

...But please, I gladly accept advice – unfortunately such clarification cannot be found on the Internet....

I would not overestimate the reliability of knowledge acquired from the Internet, mostly from anonymous hobby experts without any professional training, at best regard it as a "signpost" or orientation! On the other hand, there are enough and sufficient offers, but unfortunately not for free!

...Maybe that is a reason why nobody does it? ...

I would not say nobody, but the "stinginess is cool" mentality predominantly determines the situation and accordingly the result in each case!

...Please explain to me the benefits of this construction supervision, the approximate costs, and how to find a suitable construction supervisor....

The main benefit is that externals are not defined upfront by sales, distribution, marketing, etc., thus not involved from self-interest. These, like me, live from their own chargeable advisory and planning services, completely independent of commission payments from any manufacturers, etc.
Which professional puts knowledge and experience gained on the Internet for free on the Internet? That person would be completely incapable of doing business!
Here mostly anonymous advisors, hobby experts without any guarantee and warranty, are roaming! What is its economic and legal reliable worth – nothing! Ask your wife (lawyer)!
The costs are always specific to the construction project (effort), but certainly not unbearable!
The current construction status in the present case is unknown. After the building permit has been granted, not even a building submission authorization is required for continuation! => Costs!

...A clear cost/benefit consideration could convince many people, but as I said I can find none anywhere...

Best regards.