Making unauthorized backups of the brains (in brain chips) of the deceased and implanting them, implanting different brains than that of the original owner of the body, first steps in the experimentation of the necro soldiers technology ... There are so many possibilities of what's happening upstairs
Those are possibilities, but some of them have dubious connections to what is happening here.
The brain chips are primarily a tech from Earth, the orbital cities had a whole thing going with them, and despite the connections that exist between the planets, many of the sides that existed and exist now are/were pretty guarded with some of the things they do.
Chronologically, the brain chips could already exist now, but their use would still be limited, I think Mbady is the one that would accelerate their use.
The Necro soldiers... well, as far as I can say the Zonohedron is the org developing that tech, sure... we don't know who Dasein is or even when they may have already existed, but I think at this stage, and with some of the hints given... I don't feel like the Hexenhaus has anything to do with them.
We know the leader of the Zonohedron, and after the Grunthal's recent attack on them, I think if he had such an ace in the sleeve as an entire sect planted deep in the bowels of the Grunthal some of his reactions would have been different.
The fact the Hexenhaus planted itself right into the complex "mortuary" system of the Grunthal, they must be doing some sort of research on genetics maybe eugenics, considering how much some of the sides in this story are obsessed with those two things.
But there is one thing that came to mind with all the things you pointed out... it may be that the true scope of what the Hexenhaus did is something we can only recognize by looking at the events of the present, instead of anything major in the past.