External trade, damage repair, warranty?

but what could the tiler have done wrong?

Laid the tile surface facing down? :D

The joint pattern is not parallel to the wall, it has a deviation of 8°

That's just a visual thing, right? Is it really that bad? Are you an Audi engineer or something?

8 degrees is already quite a lot and is noticeable even in smaller rooms. A crooked joint wouldn't bother me in principle, as long as it's visually unobtrusive. But with everything else, you get eye strain.

I once tried to furnish an apartment with a room that had a consistently slanted wall (2 of the 4 corners not 90 degrees). You can't tile it; anything that runs parallel to the wall, if it were straight, just looks bad.

Photos?? Otherwise, it's the tiler's problem. Unscrew it yourself or have someone come. And as if the plumber would know that it had been removed before.

It will end up similar to how it did with us. Our parquet is a bit wavy - in every room. Taking it out and replacing it is disproportionate. We got 400 euros and the 1.5 sqm on the staircase landing had to be redone because it wobbles. That is dangerous and the rest is appearance.

Whether we pursue the rest (missing, incorrectly applied sealing, splintering wood) further, we are still unsure. This joke of a company has a right to rectification and we don’t want them in the house anymore.