Floor plan planning shortly before submitting the building application

Then he wonders why the concrete steps on one side are sometimes several centimeters less than on the other side. And he says it is not his job to finish it later. I also wonder how they still want to manage it.

what actually came out of that?

The flat roof guideline prescribes at least 2 slopes - for good reason.

In times of increasing heavy rainfall events, this point is sensitive (and the desired exemption - but only after adequate written clarification of the consequences by the planner / general contractor - is justified). By doing this, you turn your building into a guinea pig. It can go well, but it can also go wrong.

In my opinion, it says "should" there first, not "must". It's also not about protecting the flat roof from flooding, which is possible even with 0 slope, but about permanently standing water. Even that cannot be solved with 2% slope due to minor unevenness, overlapping of the roofing membranes, etc.
If slopes cannot be implemented, for whatever reason, the waterproofing must be executed with correspondingly higher quality (K1/K2). That also works.

All this, however, applies only to unused roofs. The "guidelines" for used roofs are significantly weaker.

what actually came out of that?

I am still waiting for the report. The general contractor's inspector has also been here by now and says everything is free of defects.

:
Is there a DIN standard for the parapet flashing? Then I would inquire and refer to it.

protecting the flat roof from flooding, that also works with 0 slope,

That is nonsense, or rather "works" only in theory. You will not find perfect flatness, so in practice you have puddles on the roofing felt if the entire surface does not have a slope. Zero slope simply saves work at every corner and end of any component connections – in other words: paradise for detail drawing slouchers (whether on the drafter or drawing reader side).

The expert report arrived today. Considering the hourly rate for the whole ordeal, the report does not seem professional to me and is also not really helpful for me to continue working with it...... few references to standards.....

Honestly, I can't make anything of it. What do you think of the report? Or am I being too critical?

With us, the report was clearer
Deficiencies recorded, deviation also justified with DIN
At the end, a list of who has to provide what by when
Company X proof of ..., to be provided
Company Y to repair this and that
Proof with photo
Company Z change this and that, date for acceptance

I understand, I wouldn’t be satisfied with the report either. We have an expert admitted to court and no one has objected to him. He also has a good reputation