Walls of a new single-family house, ground floor runs crooked, defect correction construction defect

Without showing off expertise. I think the OP posted the picture of the tiled storage room because it is the easiest for us to see in a photo. However, the storage room is probably his smallest problem. As he wrote and marked, the entire "staircase framing" is affected. Possibly also the wall in the living room (only this is harder to see in a photo than tiles). Here the tiler cannot compensate either – maybe the plasterer can. Whether emotionally affected or not – I can well imagine that the OP is annoyed if visitors point out an obviously crooked wall to him (without having been informed about it).

Yes, and why couldn't the tiler have noticed that himself beforehand? Didn't he have a tape measure with him? In my opinion, he just started blindly in the corner and assumed (!) that all rooms are right-angled – which he should have checked, especially with the large areas, beforehand.

I see it the same way. The tiler finished tiling the room in one day, and when I came in the evening, the ground floor was already done. This will still need to be discussed, because the two tilers definitely must have noticed it. But they just kept going – probably under time pressure to quickly get to the next construction site.
We have now scheduled an appointment with a building expert before our final inspection, who will take a look at everything and then tell us about a possible price reduction.
Actually, we regret now anyway that we didn't have an expert involved from the start.

Best regards
Markusch