Lets try again. It is very easy and very intuitive. Think again of the waves in a liquid. If we consider that the liquid has the same density, i.e. the liquid is homogeneous, the speed of the waves, say c, will always have the same magnitude in any region of the liquid. Do you agree in this? The speed of the waves is not defined by the source but by the liquid. Once a wave is generated the wave moves away from the observer at a given velocity. Ok?
Now let us make the next assumption that no ship and nothing moving through the liquid can move faster than the speed of the waves. The speed c is a limiting speed. No material object can go faster than c. ok? We can take advantage of the fact that the speed of the wave is maximum and constant in this homogenous liquid to determine the rapidity of other objects. I mean, we can take the motion of the wave as a basic unit of motion or speed, and refer all movements of physical entities (PE) to the speed of the wave. Ok? Conceive this as if it were a race between a wave and a PE. You will take as a criterion of motion the speed of a wave. Then you can evaluate the rapidity of something according to the fact that it moves slower, equal or faster than the wave.
Now, imagine that you wish to measure the speed of a (PE), say the speed v of a ship moving through the water. But you know that the ship cannot move faster than c. So, to determine the quantity of motion of the ship (i.e. how fast it moves relative to the motion of the wave). You place two buoys that will delimit a distance L. Then, you let the ship to travel the distance and measure the time it requires for the trip, say, t_s. Then you let a wave to travel the same distance and register the time, say t_l. Now keep in mind that you want to know what physical entity is faster, the wave or the ship. To make a quantitative estimate we can divide the speed of the ship by the speed of the wave. This is called beta, i.e. beta=v/c=(L/t_p)/(L/t_l). Since c is the maximum speed, beta for the particle will be less than or equal to 1. ok? This same result can be obtained if you divide only the times, i.e. beta=t_l/t_p. Do you understand my picture so far?
No, imagine that we have an inhomogeneous liquid. And this inhomogeneity is caused by the presence of a spherical source that changes the density of the liquid as function of the distance to the center of the sphere. In this case the speed of the waves will be no longer constant at given region of the liquid (do you agree?); the speed of the waves will vary, lets say, according to the expression c'=c(1+2Q/c^2). Then, imagine that you are in a given region of the liquid where the speed is not c but, lets say, 1.5c. But however in that region of the liquid, the speed of the wave is also the maximum speed that any PE can achieved. Nothing can travel faster than 1.5c. Therefore, if you would like to measure the speed of any PE according to the procedure above, you will obtain again that beta=v/1.5c is less than or equal to 1, but in this case v has as a limiting speed 1.5c and not c as in the case where the liquid is homogeneous. If you further go to another region, there you will find another absolute value for the speed of the wave, and again in that region the speed will be the maximum speed, in that region nothing can travel faster than the speed of the wave. Thus is you measure the speed of the wave, making reference to the speed of the wave you will get again 1.
Now translate these ideas into the case of the speed of light and keep in mind that light travels through the aether (free space or vacuum). In a homogeneous aether the speed of light will be always c. But under the influence of gravitational fields the aether is inhomogeneous and the speed of light is no longer c. Nevertheless, in a given region of space, it is the maximum speed any PE can attain, then, if you follow the experimental procedure outlined above to measure the speed of light, you will get a constant value, i.e. beta= 1. The same value is obtained both with a gravitational field or without it. This is so, because the speed of light at a given region of space is the maximum speed. Do you understand this? Do you understand why the speed of light is constant when it is measured although it is not in reality as it moves from region to region in a gravitational field?
Now, the next thing you have to do to fully understand these ideas is to read my reference 17. There I explain that the experimental techniques that we use to measure the speed of any PE are incapable of measuring the speed of any PE in one way. We can only measure average speeds, not instantaneous speeds. To the best of my knowledge, measurements of the one-way speed have not been possible so far.
I agree with the quote of Steve Carlip in your last post. But you should understand that theory and experiment are two different things.
I hope that this time I have elucidated this issue. Please let me know you got it.