Is an offer binding or are deviations allowed?

I find single awarding difficult. Who requested all the technical details? The construction supervisor or you? I would never trust myself to do that!

Which details in particular do you mean, for example?

As explained, the construction supervisor is responsible for that...

The rest was common sense regarding the planning itself.....

And I am sure: with the general contractor, we wouldn’t have even questioned what is on the roof.....

The great thing about the general contractor (GU) is that there is EXACTLY one contact person. No matter what the subcontractors mess up, the contact person is the GU. With individual contracts, two parties shift the blame and you can settle it in court. With the GU, that is not possible because he is always contractually the contact person. Now that's just how it is for the time being. And again: what does your expert say? That he initially does not count in court is irrelevant right now. The point is to find a solution now. He knows it, he knows which section the matter belongs to or where the error lies. Otherwise, that is simply the risk of architect-led construction compared to construction by a GU.

With the general contractor, this is not possible because he is always contractually the contact person.
Now it is simply like that

and this is how the general contractor excuses himself, who has much more "power" than an individual trade. I don't know what is better.

Then this would have to be stated in writing somewhere that you specifically want to deviate from it....

Exactly, now we are going around in circles. What was the basis for the offer/order/assignment of the carpenter? A work or execution plan? A specification? Was this deviated from and did you approve the deviation? If the ordered target from the order letter/work plan/specification deviates from the performance provided, there is a defect. This defect is reported, with an appropriate deadline for rectification. Did you even accept the roof?

and that's how the general contractor gets out of it, who has much more "power" than a single trade. I don't know what's better.

he can't make excuses like "not my problem" because he is responsible for all aspects of the ordered work.
we had one case after moving in. one roller shutter didn't work, which couldn't be seen during handover because the electric shutters were only put into operation afterwards. contacted the window manufacturer "roller shutter not functioning." window manufacturer sent the producer, who then said there was no power to the roller shutter. contacted electrician, who then said there was power at the handover junction box.
logically, the cable from the junction box to the roller shutter was broken (first the stupid question: why doesn't the roller shutter manufacturer just replace it? he should have seen/measured it...)
contacted window manufacturer that he should do something – answer: I'm not responsible for the electrics
contacted electrician – I'm only responsible up to the junction box
well, no one wanted to commission the system

so: contacted the general contractor with window manufacturer and electrician in CC: roller shutter not functioning, cable defective between junction box and roller shutter. both subs refuse to fix the defect. please do something...
2 weeks later the window manufacturer showed up and replaced the cable (carpenter).
the whole story took so long that I first installed a pleated blind on the window...