The optical resonator experiments proved the isotropy of light frequency and not the isotropy of light speed (null result of MM-experiment).
So the isotropy of light speed is an invalid conclusion of these tests. For this conclusion it has to be assumed that if there is no change in frequency there is also no change in speed of light.
At the end you have nothing more than the conclusion that the light speed is constant when the light speed is constant.
The assumption that for resonator experiments the isotropy of frequency is equivalent to isotropy of speed is extremely weak, because there are several examples which show differences in speed at constant frequency:
* light speed in ordinary matter (is generally accepted)
* light speed in vacuum at different gravitational forces (shown by several observations like gravitational lense effect, stellar aberration, stellar spectra)
* sonic speed (Doppler effect, is generally accepted)
Another inconsistency of modern physics is that the isotropy for speed of light has been disproved by Sagnac effect (Michelson-Gale experiment).
The VTOE proves and provides scientific explanation that the physical laws for speed of light in vacuum are identical to those for speed of light in ordinary matter. In both cases the energy of light quanta are transferred by dark matter, thus by the identical physical mechanism. Concerning speed and frequencies there is also an analogy to sound.
The basic scientific principles, particularly the principle of relativity, are generally valid.