I agree with almost all the aspects you have pointed out. Anything I write has its basis in Hierarchy of Energy theory, the book I mentioned above. While I said the speed of light is measured the same when we measure it within a reference frame. That doesn't mean I consider the speed of light is the same when comparing it between one reference frame to another. Hierarchy of energy theory coined the phrase "Relative Constant". It refers to light always measured at c from within a reference frame BUT when comparing reference frames, the speed of light is relatively different. I think you support that notion.
In hierarchy of energy theory, spacetime's composition is proposed as a medium of its building block component and the energy required for their alignment. Spacetime is essentially an energy medium and energy creates distance. The relative Movement of spacetime along a direction means additional energy along that direction and the quantum distance relatively shortens. Since the speed of light in hierarchy of energy theory is always defined as 1 quantum distance / quantum time unit, the speed of light will relatively shorten in spacetime we are moving through relative to when we are not moving through... the problem is we can never measure it. AND that is hierarchy of energy theory's account of the MMX results.
I do agree that Spacetime is not made of nothing...
This next statement should raise an eyebrow... Light does carries energy and it does so just the way we say it does... E = hf The energy of light and the energy of spacetime represent the base energy tier and light the next higher energy tier. Once particles of light were made at the big bang (they were made first), they make a wave in spacetime's energy. however the energy of each MUST remain independent (hence hierarchy of energy). if one could still become the energy of the other, the photon would eventually cease to exist. The fact that the transition between these two energy tiers is no longer possible is the basis for the law of conservation of energy.
I think you would be fascinated by the mechanism behind hierarchy of energy's model. I really can't go into the math detail here but it's in the book and I think you will enjoy the book when it comes out...
All the best John