I don't really agree that "Unique Prime Plane" has little to do with Planescape. Overall the Prime is greatly ignored in PS, but its an important part of the setting. Why is travelling to the Inner planes Planescape, travelling to the Outer Planes Planescape, travelling to the Transitive planes Planescape, but travelling to the Prime isn't?
Where does it say that ghosts are linked to the Astral plane? And even if they were, why is that such a bad idea? Afterall, the Astral lies between this world and the next... but eh, whateva.