I have tried those double designspace corners, because it was even worse before I had them (but then I also had 4 more masters). And also I thought that this is what Georg meant by fully cutting through the designspace. But maybe I misunderstood.
Also, you are right, the intermediate layer instances behave weirdly. And the designs are not correct. It seems, the intermediate layers are based on other intermediate layers and not on the masters. When I re-interpolate all the layers multiple times , they keep changing.