Fixed price guarantee 12M at risk due to capacity - experiences after lawsuit?

For the OP, it may be about a five-figure sum, so a valid assistance is worth something. To what extent the talks with the construction partners have been exhausted and whether they were fruitful is unclear.

For us, not only money is at stake, but also the threat of an eviction from our previous apartment.
The manufacturer is aware of this. Probably nothing more can be argued about it. If still no other offer comes than the categorical refusal to hand over at least the construction plans of the basement, I consider the matter "exhausted."
For me, it is now interesting whether there is hope to prevail in a legal dispute, whether perhaps just sending a lawyer in advance triggers the desired reaction (as in the case of motor vehicle damage compensation, where there is generally an attempt to settle with a lesser compensation) or whether I am just throwing good money after bad here.

Stupid messed-up situation, especially since you have real time pressure that your construction partner is ignoring. A loss of scoring points in the construction legal protection insurance would only cause me minor concerns. Let them help you, after all, that's why you took out this insurance. It's important after all!

Are you sure that you have [Baurechtschutz]?

Construction legal protection insurance, as far as I know, is only available from two insurance companies that end with ...RAG.

As far as I know, construction legal protection insurance is only available from two insurers that end with ...RAG.

Yes, I have one of them. :)

A decision-maker at the manufacturer is currently ill. Possibly the representatives are also afraid to make a decision that goes beyond routine and for which they lack the competence. I want to wait and see about that.

If there had even been one person here who had taken the legal route like in my case, it only takes a few minutes to get oriented. Not a big waste of time.

By the way, I am still a member of two associations: the Bauherren-Schutzbund and the Property Owners' Association. Maybe one of you has ever sought advice there?

or if I am throwing good money after bad here.

Oh that happens very quickly, and maybe even in all scenarios, without you being able to prevent it now. Only minimize.

It’s not just money at stake for us, but also a threatened eviction from our current apartment.

I can only recommend already looking for and renting a replacement apartment and/or finding a way to store the unimportant belongings. That also improves your negotiating position with the contractor if he knows that the severe time pressure on you is gone and you don’t have to swallow every toad anymore.
Even more important than the costs is that the mental pressure and the time pressure are taken away. More valuable than gold. Something practical and very small is of course enough.