This assumes the light we see today represents the energy at the light's origin. If the the speed changes it would assume that the energy it represents would change with it along it's travels (if all laws decay together). Whether the origin had more or less energy than it does today, assuming all laws of physics are changing as "time" and "space" change, then would the results not remain consistent (red shift)?
In deep space we measure mass and energy based on gravity (and how light is affected as it travels). If the energy of the light that has been traveling for billions of years decayed at the same rate as the speed, and at the same rate of the origin's mass and energy, would the red shift not show what we see today? Remember, we assume gravity as a constant function of mass and space throughout the universe too. Yet we do not understand the source/force that causes magnetic attraction associated with mass. If the laws of gravity have evolved along with energy, speed of light and mass traveling in the universe, then our snapshot is accurate but not representative of Universal Laws of Physics at the moment of the Big Bang.
For that matter, what if gravity as a function of mass is different in every galaxy? We can only assume the consistencies beyond a certain distance and speed as we rely on light and use that data based on established Laws of Physics. The very mass of our own planet is established based on it's own gravity and it's consistency with that of our moon and surrounding planets. These functions are observable in our own solar system and establish our understanding of the mass of our sun and our neighboring planets. But Satellites verify the size of our planet based on the known distance of our moon based on bouncing lasers off of solid objects. We can't bounce lasers off of distant stars (and galaxies) so we rely on the light they sent to us billions of years ago.
Again, I assume you to be of superior knowledge on this subject, but am fascinated with Science's inability to reconcile our Laws of Physics with what is theorized as our Universe's origin. I recall learning some Physicists frustrated conclusion that the Physics to support the Big Bang Theory do not fit the Laws established today. As a neophyte, this has always begged the question for me, "Why do we assume the laws are constant over billions of years?" Couple that with recent revelations that Quantum Physics is finding particles that don't obey our established laws and I say it is fair to explore taboo ideas that go against the established Laws of Physics. After all, is that not what science is?
I've had this discussion before (likely with less knowledgeable subjects) where someone finally said, ALL scientists agree..... CONSENSUS? LOL Ultimately, I giggle at the ambitious and sometimes presumptuous human effort to figure out how God did it. But I admit to being curious myself and I think God programmed us to be this way.
Thank you for your effort, I am learning a lot.
Then we'd be able to see the galaxies behaving in different ways despite similar masses.
What you're proposing is not one or two laws of physics being in flux--it's all of them being in flux in such a way that the net effect is exactly zero in terms of stellar luminosity, gravitational attraction, etc. That's not only a gross violation of Occam's Razor, it also makes no sense from a theistic viewpoint: If we believe in One God as both the Creator and the Lawgiver, and we believe that He is unchanging and constant, why would we hypothesize that He is constantly changing the laws of reality in a way that we can't even observe the effects?
There's a similar problem for those who posit that God created the light of distant stars and galaxies in transit to give the universe an appearance of age: Quite aside from the very obvious question, "Why bother?" that means that nothing outside of 6-10,000 light-years actually exists, and God made the universe to lie to us. That is, when we see the light of a supernova in the next galaxy over, the star it's apparently coming from never existed, and it's all a lie.
Either way, you end up with a "universe" that is either Lovecraftian or the Matrix, and a God very different from the One who presents Himself in the Bible.
I don't think God is deceptive. I do think that it's possible to misinterpret both the record of nature and the words of the Bible, but I don't think that the Lawgiver gave us constantly shifting laws just to mess with us.
Shalom