Thanks for posting this link. Indeed, Einstein very much states that an ethereal concept is required however we cannot ascribe any concept of motion to it. The concept of motion has no meaning in space - or in an ether concept - as I already conjectured in my previous reply. Motion is an object based construct.

Its not the ether Einstein is objecting against. Its the "stationariness" of the ether.We cannot ascribe either motion or stationariness to the ether.

Is the speed of light a constant or is it that we always measure it as a constant since any velocity we have as we measure it would affect the clock we are holding to measure it and thus change the V=d/t equation so that the answer is always the same?

Dark matter and dark energy may allow us to see light travel in a medium. This slows the light.

As far as I remember, Einstein just (after briefly mentioning failed experiments to find the ether) postulated c to be constant in any reference frame and then looked what that would result into. For me the most important was that he found symetric equations for the motion of a magnet, where they weren't before that postulate. In the end it was just his genius to see that if we can't find that ether that until now (1905) we needed to explain some phenomena, what would happen if there is none. And it turned out that his results offered much simpler explanations, especially symetric ones for the same observations in different frames than before. At the price of some real strange features of space and times of course.