ᐅ Shower drain installed incorrectly – Should it be accepted or not?

It depends on the price/the reimbursement. I wouldn't care, but I also hang posters crooked.

Is it normal for the gutter to be flush against the back wall? I am used to seeing about a 5cm (2 inch) gap at the back...

I would consider it too risky that after repositioning, everything might no longer be sealed properly, that the underfloor heating could be damaged if the screed needs to be broken out, or that some other technical issue might occur.
Yes, it looks bad. But leaks would be even worse.

Besides, does a company have to perform repairs that are "disproportionate"? It’s "only" a cosmetic defect, which already causes a few hundred euros in costs just from removing and reinstalling the tiles. Then probably work on the screed and realigning the pipes... I can’t imagine someone would win a case over this...?

Someone once wrote that in the case of a purely cosmetic defect (depending on its severity), the claimant is only entitled to a certain percentage of the correction costs. In other words, the contractor is not necessarily obliged to fix it at their own expense. That actually makes sense to me... even though it's frustrating for the client.

I find it odd that the gutter connects directly to the wall; I’m not familiar with that. I don’t know if this could have any negative consequences.

The risk of poor workmanship when making corrections is definitely high. However, I would also be very annoyed having to look at it every day. Therefore, try to negotiate as high a compensation as possible. This just seems like a really unfortunate situation.
That the tiler didn’t notice this while laying the tiles is absolutely incomprehensible to me.

I am generally quite a picky person and I prefer things to be straightforward and precise. However, I have to say that this visual flaw in the shower wouldn’t really bother me that much.

That said, it is still frustrating, no question.