Yes that's it !
Thanks Dune
Using the Vector Displacement as the child of a surface shader and blending back in a single Add Vector using blend as coverage allowed eliminating all of the extra nodes and keeps the planet at 0 altitude.
Just need one Add Vector now as Matt said. ( I have the Add Vector is set -0.46,-0.46,-0.46, which seems a little strange but that is
the exact amount needed to bring the displacement level. At least it is far closer to the expected -0.5)
A Plus is that the displacement has fewer distortions from what was modeled .