So after giving your 930 to a "subcontractor" who is actually doing the work now since they admitted they didn't have the expertise?
It sounds like you are close to a resolution but I would still make Porsche HQ in Reading aware of your experiences.

In future, I would maybe investigate having your 930 maintained by someone like JZM (Jonas) or Steve Winter at Jaz Porsche.
I had 3.0 SC, 3.2 Carrera and 993 all looked after by them.
It would be a very long drive but it would be worth it.

Good luck