Everything you have mentioned had played role developing Special Relativity.
Besides the fact, that measured by many experiments velocity of round - trip of light appears to be always c, other details must be considered, because this phenomena according to different theories has different explanation.
A. Einstein once noted: “Concerning the experiment of Michelson and Morley, H. A. Lorentz showed that the result obtained at least does not contradict the theory of an ether at rest.” http://www.relativitybook.com/resources/Einstein_space.html
I will express my opinion, I believe that some people may not agree.
I believe that this theory in Einstein's incarnation is a bit muddle (again, this is my opinion), though that absolutely does not mean, that formalism of this theory is wrong.
It should be noted, that Poincare and Lorentz before Einstein had mainly developed formalism of the theory. However, you will not find any reference in Einstein's 1905 paper.
Einstein in his second postulate simply says, that "light is always propagated in empty space with a definite velocity c which is independent of the state of motion of the emitting body"
However, speed of waves on the surface of the pond does not depend on
speed of your boat too, if you drop a stone into water. Though speed of waves relatively to the boat depend on the speed of the boat. That doesn’t seem very curious.
“Real” Einstein relativity with all its wonders comes when he speaks about clock synchronization. In relativity we conduct measurements by means of rods and synchronized clocks. Synchronization procedure is the essence of the theory, because outcome of your measurements would depend on synchronization procedure. Einstein proposes to synchronize two spatially separated clocks, that are at rest relatively to each other in a chosen reference frame by the same method, assuming that one way speed of light between these clock is c.
It is not correct to say, that experiments completely ruled out Ether theory. The first person, who explained null – result of MM experiment was H. A. Lorentz. According to Lorentz, measuring rod of moving in the Ether observer’s contracts in the direction of motion and his dilated clock shows not real “absolute” time, but so – called ”local time”. Lorentz’s theory predicts null result of any experiment which intends to detect absolute motion. In Lorentz theory, one way speed of light is isotropic only in absolute frame, or Ether.
Many researchers went through hundreds of different setups of experiments.
However, if you bear in mind that you clock dilated and measuring rod contracts you can predict, that these experiments would not be able to detect your motion through the Ether.
According to Lorentz, one – way speed of light in moving frame is different in different directions, we have to take into account velocity of laboratory in the Ether. However, since an observer cannot detect his own motion through the Ether, he doesn't know how to synchronize our clocks to measure one - way speed of light in our Earth. For example, he may consider himself as being "at rest" and synchronize his clocks assuming, that one way speed of light is isotropic in his frame.
Let's consider the following example: you have two clocks side by side. You and your laboratory on Earth move in Ether. The clocks are synchronized and their hands show 0. You stay are in the middle of the room and throw these clocks in opposite directions by identical cannons. They travel equal distances and stop (for example hit walls). Then you send a beam of light from one clock to the other and measure one - way speed of light. However, one clock moves a bit faster in the Ether and another moves a bit slower. Since they dilate at different magnitude, due to this effect clocks will be out of sync after they had stopped and measured by means of these clocks one- way speed of light will be exactly c.
Lorentz and Poincare deduced transformations from stationary frame into moving frame. These transformations had been named as Lorentz transformations.
Good to note, that in Lorentz theory any observer can erroneously consider himself as being at rest and synchronize clocks in his frame assuming that one - way speed of light is isotropic.
Moving observer is actually "slower and shorter", than that one, who is at rest in the Ether. However, if each observer synchronizes clocks in his frame assuming that it is he who is "at rest" (one way speed of light is c, Einstein synchronization), each of them will see, that it is the "other guy” who is $\gamma$ times slower and shorter.
Lorentz theory is empirically equivalent to Special Relativity, it predicts the same outcome of all experiments and the same paradoxes and symmetry (reciprocity of Lorentz transformations). However, in Lorentz theory this symmetry is due to that reason, that all observers rule out their own motion and consider themselves being “at rest”.
Again, in Lorentz theory you clock dilate and rod contracts due to your own absolute motion, thus you cannot detect your own motion and anisotropy of light.
In Lorentz theory velocity of light as measured by any observer with a single clock “back and forth” is c in any inertial frame. In regard of Michelson - Morley experiment we have to bear in mind, that times of round - trip of light along different shoulders of interferometer in moving frame appear to be the same, since longitudinal shoulder $\gamma$ times contracts.
Einstein found a simple answer. He assumed, that absolute motion does not exist and one – way speed of light is c in all frames (every observer is at rest in his own frame) and thus, since synchronization procedure of spatially separated clocks is the same in all frames, that leads to so called relativity of simultaneity. Due to relativity of simultaneity you will always see, that it is not you, but the “the other relatively moving guy" is shorter and slower.
Some people say, that Lorentz contraction hypothesis is ad hoc. It is diffucul to understand this claim. Even in Special Relativity measuring rod that rotates around the sun contracts in the frame of the Sun. We can imagine a pinhole camera, that rotates around Sun. Due to Lorentz - contraction of the film of the camera, the Sun will appear stretched on the film. If it appeared stretched, that means the film had actually been contracted.
Einsteins hypothesis seemed to be a remedy from any headache (though creates new ones, for example in rotating frames). You needn’t think about dynamical explanations for Lorentz contraction or time dilation. You don't need to think about properties of medium. Null – result of MM experiment? Because you are at rest. All frames are "the same" and one way velocity of light in all frames is c!
So, when Einstein speaks about clock synchronization, he assumes, that one – way speed of light is c an all frames (Einstein synchronization).
Supporters of this theory rule out absolute motion. They consider Minkowski space – time as a “source” of relativistic phenomena. When it comes to explain twin paradox, they draw diagrams on a piece of paper, though in Lorentz theory resolution of the paradox is very simple. Even kids in elementary school can calculate it. Minkowski space - time, (in some sense) in turn is a consequence of relativity of simultaneity.
You can find some info about Einstein synchronization and one - way speed of light here:
Very good paper (though it doesn’t reveal certain details) is by M. Janssen:
A Comparison between Lorentz's Ether Theory and Special Relativity in the Light of the Experiments of Trouton and Noble.
To make conscious choice whether to accept or not to accept axiom about equality of speed of one way speed of light in all frames (Einstein synchronization) it is good to look through different phenomena through the prism of different theories.
This article – Simulation of Kinematics of Special Relativity by means of classical mechanics simulates all effects and paradoxes at turtle’s speed in a water – time dilation, length contraction, Lorentz transformation, relativistic velocity addition, twin paradox, Bell’s spaceship paradox, symmetry (barn and ladder paradox), Minkowski space time. In this model exists medium, absolute time and anisotropic velocity of information carriers in different inertial frames.
It doesn't introduce any ad-hoc hypotheses. It shows that "space - time" of moving barges is elementary mathematical construct that pertains more to the formalization of the measurement errors caused by the failure to take the presence of an aquatic environment into account than to the behavioral features of the barges
on the water surface.
Note: there are some thought experiments (with transverse Doppler shifts due to aberration of light in relatively moving frames) that clearly show, that speed of light in different frames cannot be c, but it is another story.