Construction progress: Duplex with WU basement and developed attic

That is exactly your problem, you want to listen to people you pay. We are a construction forum here and every single person has told you that it is not possible. Unfortunately, you keep getting the consequences for precisely that unreflective behavior. We have all built and know the prices. My architect calculated pure construction costs of 400€ per cubic meter including the outer shell and ceilings three years ago.

GU2 has to fix his defects. Either through his SUBs (if he can still reach them) or he has to cover it himself. He is legally obliged to do so and cannot get out of it. Since he caused a lot himself and there are no real contracts with the SUBs, he will probably be bankrupt afterwards and unable to complete the work. Then his contract fulfillment bond will be called in first. I still have enough money myself for the continuation of the construction. So that would not be the problem. It is important to determine the exact amount of the defect correction costs.

Okay then you definitely have the means to continue building. Let's see where you end up in the end.

I mean GU2 won't be able to fix the defects. That has already been shown.

Not even himself, we would be back to the same point. But he has to finance the defect rectification.

Yes, but if he is bankrupt because he completely miscalculated and overextended himself, then he possibly will not be able to finance it.

Then according to lawyers (his own and the opposing party's) he has to mortgage or sell his properties.