Another point here. Take a light wave today. Its speed c is a product of its wavelength and frequency, since you can think of it as a little sine wave flying by you with peaks and valleys. Each peak/valley pair going by is a "cycle." The "cycles-per-second" (frequency) times the physical length of a cycle must yield the overall speed of the stream of ups and downs.
Now imagine it's a Barry Setterfield universe and it's 6000 years ago. The stream is flying by 11 million times faster than the modern value. Your proposal is that the wavelength of the light is unchanged. That forces the frequency up (way up), making all the solar photons ultra-blue. (Probably gamma ray but I'm too lazy to check.)
In fact, Setterfield went the other way, deciding that solar photons are redder, not bluer. (This happens because all the photon generating reactions are operating with less mass. The universe is light as a feather but you can't tell because gravity is very much stronger than now.)
Anyway, your version still has the same problem as Setterfield's. If Adam is in the Garden of Eden on Day 6, he can't see the solar photons. His eyes are tuned to the wrong part of the band. He's blind. And he's cooking.
Except I don't think X-ray and above penetrate the atmosphere too well. So maybe he's freezing, except he's got a real storm of alpha particles from nuclear reactions in the earth to keep him warm.
I'm lazier than you, but I think you'd have cosmic rays. Whatever.
When you are refering to gamma radiation, or ultra-blue photon's you're referring to wavelenght. Inherently in the equation is a frequency based on some arbitrary value of 'c'..
It would appear, nonetheless, that changing the speed of light could have an adverse impact on optics, in that the relative index of refraction would be changed. However, according to Snell's law it is the ratio of the velocities of an incident waveform and the velocity of the waveform transmitted through a different medium (part of the incident waveform being reflected). What evidence is there that the relative index of refraction wouldn't be the same regardless of 'c'? We know that the wave frequencies of both incident and transmitted waveforms are identical, but their wavelengths change. Does this mean that if 'c' itself changes, that the relative index of refraction needs to change somehow?
If that is not so, why would Adam be blind if the speed of light (and associated frequency was higher)? Have you gone through the math yourself?
Furthermore, I see no issue with regards to functionality of diffraction with respect to the speed of light (frequency plays no part in it). Constructive and destructive interference also do not seem to be affected by differing frequencies (that behavior of light is entirely predicated on the wavelength of the light. When one examines polarization of light, it pertains to transverse electric vector viewed along the direction of the waveform propagation - polarization by reflection, refraction, selective absorption or scattering - so again frequency doesn't come into play.
Ah ha! You'll state that the frequency of light will affect Rayleigh Scattering (why the sky is blue). The closer the frequency of of the light wave is to the natural frequency of the electron of the predominant gas in the atmosphere (nitrogen), the greater the amplitude of vibration and the greater the scattering of the light wave. Thus the components of light having shorter wavelengths are scattered moreso than those with longer wavelengths. It has been shown that this scattering is proportional to the inverse of lambda to the fourth power. Even so, it appears that light is scattered according to wavelength, it actually occurs as a property of incident light (frequency) and interspersed atom's natural electon resonance (frequency). In accordance with Setterfields postulation, the fundemental resonant frequency of an atom's electrons is proportional to 'c'. Basicly I don't see a problem. Back to your Evolution pamphlets.