"speed of light" is the speed in empty space, without any obstructions.

It is already known that any obstruction, reflection or other influence on the light can increase the time it takes to cover a specific distance. But if you would track the exact path it took, you'd get back to the speed of light.

But one of the primary ideas of the scientific method is that only the data from experiments matters. No value is unchangeable.

Since every change to the overall model needs to be consistent with all other data we have gathered and all previous experiments have to be explained as good or better than the previous, every change will only make the model better.

As a principle, if it does not correlate with the data, it is not a valid theory. Theories are "facts" that only are considered facts because all attempts to disprove it have failed.