This is not an attempt at a full response, but perhaps a few points can be cleared up.

Quote:
the pressure from overburden is important throughout the entire mantle


Agreed, but I did say “less important”, not unimportant.

Obviously, bodies of rock can move vertically with respect to surrounding rock, without melting, otherwise it would not be possible to maintain isostatic equilibrium.


How do you explain the apparently descending bodies of rock with increased seismic wave velocities detected by seismic tomography in what current thinking identifies as subduction zones?

Out of time, unfortunately. More later.

There never was nothing.