Fred,

If you think about the theory, it does presume space to expand, relative to the speed of light. That as it expands, light takes longer to cross it. That eventually it will expand so much that we can no longer see any distant galaxies, because the light can no longer travel as fast. That means the light speed is still being used as the Ruler!!!!! The denominator. The metric against which this expansion is being judged/measured!

It doesn't invalidate relativity, it's simply incompatible with it. Relativity invalidates it.

The speed of light is not constant to the frame, so either the light is slowing relative to the frame, or the frame is expanding relative to the speed of the light.

Since it's not a "tired light" theory, it means the frame is expanding relative to the light.

So either we are back where we started and we must be at the center of the universe, with everything traveling directly away from us, or redshift is an optical effect, compounding on itself. Thus the parabolic curve in the rate, currently requiring Dark Energy to explain.

One way light does redshift over distance, rather than recession, is as multi spectrum "packets," as the higher frequencies dissipate faster, but that would mean we are sampling a wave front, not observing individual photons traveling billions of years. Which creates complications for quantum theory.