I think at this point you should ask yourself, what result do you really expect to see? From the last examples, i can clearly see that displacement is working as expected - between support loops, it is displacing strictly perpendicular to the surface and between the edges and support loops, displacement direction is gradually leaning towards the edges to flush meet with the other surface. Alternative to this, would be big gaps in mesh at the hard edges, that's how Corona displacement used to work many versions ago and nobody liked it.