Incorrect construction method: Drywall and insulation from Knauf?

Then I was basically lucky to have gotten someone who not only had one, made it part of the contract, and also largely stuck to it? Wow.

Hello,

Profiles from Knauf CW75 (75mm) and a mineral wool insulation from Knauf (40mm) are used. According to Knauf, these must be installed slip-resistant because they do not fill the cavity to 80%

So the drywall company is using an insulation that does not fully fill the cavity in the cross-section? I cannot assess what you are owed, but I would not build it privately.

On the subject of sheathing: As a pure partition wall, it can be sheathed on one side only. Two-sided sheathing is recommended where loads are expected. But even then, at least where something is screwed on (kitchen, bathroom radiator etc.) an OSB board should be attached as reinforcement. A full-surface covering with an OSB board is of course nicer, but more expensive for you and not absolutely necessary if the specified heights are adhered to.

We also find it strange that the drywall walls in the bathrooms have an 18mm gypsum board inside and 12.5mm on the back side.

Regards Infuso00

Regarding the different board thickness: Is the board in the bathroom green?

To assess your concern: I believe that with this construction, airborne sound transmission of bathroom noises is possible.
As for the water pipes, it is more important that there are no impact points and that the pipes are well insulated and soundproof pipes are used (this is indicated on the pipes). Of course, the pipe must not have contact with the wall, that would be fatal.
With a 75mm U-profile there usually is not much space in the wall for pipes. At most drinking water and DN50 wastewater from the sink.

I will send a picture shortly showing how we solve bathroom walls here (but it is always a question of budget).

I am not familiar with construction developer law to be able to say which steps you can take now.

Whether it is permitted to build like this or not must in any case be assessed by whether the generally accepted rules of technology are implemented. A DIN standard, for example, tends to represent these but does not have to. However, the manufacturer's specifications must be implemented. If these are ignored, it will be difficult to argue that the generally accepted rules of technology are still being followed.

CORRECTION:

I can no longer edit my previous message, so here is a correction:

I was a bit too hasty with the statement that a 40mm insulation in a 75mm profile is too thin. It depends on the product!
I just inserted a 40mm panel into a 75mm profile and the panel fills the cavity very well. The insulation holds by itself.

I hereby take back my statement that you should expect noise problems. Of course, the single-layer cladding lacks some mass, but I do not want to judge the resulting sound issues. Just this much to alleviate the concern, if the insulation looks like in the picture, it is okay in my understanding. I would not object to it. There also have to be boxes and cables in the wall.

In addition, I photographed the case where a 40mm panel is in a 100mm profile. There you can see that the cavity is not filled. I will, for example, report this as a defect and have it fixed (No, that was not a coincidence, you usually notice something like this before a "wall and shaft acceptance" before closing).

Then I was basically lucky to have gotten one who not only had one, made it part of the contract, and also largely adhered to it?

If the construction service description has become part of the contract, you have to adhere to it. However, this is usually the case with the general contractor (GU) yes and with the subcontractor (BT) no.