Slanted window interior and too low room height?

Yes, thanks first of all. : sorry, you are right. I was too eager.

Since the ceiling height obviously cannot be increased any further now, the question would be how to proceed appropriately. As a result, presumably only a notice of defects, possibly combined with a deadline of 3 weeks for remediation (although not really feasible), would be an option, which would effectively amount to some kind of compensation payment. I mean, tearing the house down and rebuilding it is not a viable option - for either side.

I am also somewhat facing the problem that the final acceptance (i.e. still 16% VAT) is planned/was planned for December.

First, make sure that clear height with reference to the finished floor was actually meant. It could well be that the raw floor was meant, and then you would even have plenty of space...
In addition, for a possibly justified defect notification, the comparison with the construction service description is crucial. Only if it states that the room heights are derived from the floor plan and the measurements given there are binding, can you raise a defect.
Is it even allowed to build below 2.5m in your country? In Berlin, that would no longer be living space on the ground floor... Maybe you can get around that.

The clear height/room height is ALWAYS from the top edge of the floor covering to the bottom edge of the ceiling slab! If the ceiling is suspended, then the bottom edge of the suspended ceiling serves as the measuring point. This is a defect, because with the floor covering you get (if at all) 2.45m! And that the screed will settle by 3cm in 3 years is not only wrong, but would also be fatal!

Replace IMMER with usually and I am with I am with you. I have already read construction service descriptions where at one point clear room height was stated and only later clear structural height.

no, there is nothing to discuss! Clear room height is precisely defined! The clear raw construction height is ALWAYS the top edge of the raw floor and the bottom edge of the raw ceiling and thus differs from the clear room height!

Then ALWAYS replace halt with definitionsgemäß. I was only concerned that it is used inconsistently. And that may then also be wrong. Only the ground floor of the finished house doesn't suddenly become higher because of that. Of course, you probably have good chances of financial compensation, but how high that has to be, I would now be at a loss.