Error in the static calculation, supposedly our mistake

[...] Apart from that, it is basically proper to conduct an on-site inspection during modifications in order to determine measurements and deviations from the planning documents on site.

Well, he was satisfied with the construction drawings and did it remotely. Well, your statement basically confirms everything again. As private individuals, we didn't know, and he only did the bare minimum. We just weren't used to that from the other trades, they were close to us in every situation.

And then there are construction companies that say: load-bearing wall, we'll just put a steel beam in. Which one? Let's take a look at the datasheet and, if in doubt, use a thicker one. Ok, that works.

Structural engineer? Waste of money! I'm not saying it's good, but it seems to work.