With the aid of some helpful person on a Qt forum I finally found a solution for my problem of not being able to generate a QObject based singleton in a Windows DLL:
Now with this solution at hand I can say that “somehow” also this current question is solved, or better: not any more relevant.
Because instead of using the “manager” facility of ParaView I can now generate singleton “manager” classes that are not plugged into that manager facility at all, so also no need to properly link to vtkApplicationCore.
At the same time also my previous workaround (a non VTK/PV/Qt singleton) is obsolete.
Solution: see the link above