OMG thank youI'm going to sit down and read your whole proposition either tonight or tomorrow if all goes well.
For now: Einstein started with the equivalence of gravitation and inertia when he derived GR. The equivalence has (yet) to be proven, though it has passed many tests indirectly with the testing of GR. I don't know if experiements have ever been done on this directly. I would think so but I don't know. It's a bit like Einstein assuming that the speed of light is a constant in any inertial reference frame. It was an uncertain proposition so it too needed some confirmation. We still haven't really proven it but it is now accepted. No one in the professional Physics community really has any doubts about the constancy of the speed of light. The equivalence principle in GR falls under the same heading.