That's the danger with words Bahama.
And why mathematicians would prefer us all to to use symbols and numbers.
All velocities define, as you say, some value and a direction. A speed only defines a value. When we define 'c' we speak of the 'speed of light in a vacuum' not the 'velocity'. Which means that the speed of something bouncing between mirrors can be constant while its velocity only can be the distance of one bounce, with the next bounce defining a new velocity, even if it is the exact same (speed).
There is one more reason for using the word speed as I see it. We can only measure light as it annihilate, as far as I know. That means that we cannot assume that it 'move' in the same way we can describe a car to 'move' and so have a velocity. That we find a source and a sink connect that 'photon' doesn't, to me that is, prove its existence at all those other points of its presumed 'propagation'.
But there you will find a lot of people disagreeing with me :)
The guy that wrote "As measured in any inertial frame of reference, light is always propagated in empty space with a definite velocity c that is independent of the state of motion of the emitting body." was, as I think, of the distinct opinion that light 'propagates', and that you therefore can assign it a velocity, indicating a direction as well as a speed inside our 'room time'
And it's also a function of gravity, gravity defines SpaceTimes 'straight lines', just as Earths 'roundness' defines the shortest line for you walking between two points. But gravity is trickier as it is in three dimensions, whilst the surface of our Earth could be described as being in only two as I think of it, like a bent paper.
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"As measured in any inertial frame of reference" is not necessary as I see it, 'locally' (your frame of reference, as your retina) I know no way you ever can measure 'c' as being of another speed than 'c', be it in a acceleration or in a uniform motion. A inertial frame of reference is one that is 'at rest' aka 'uniformly moving'. A planet may not fit that as it is 'gravitationally accelerating' but we still use Earth as a inertial frame. It's somewhat of a matter of taste that one. But it's tricky, as most definitions in physics.
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Rethinking that one, maybe it's simpler to use a uniformly moving frame as a reference as when we measure a speed always will need a distance, measured in time. So yes, it makes sense as a accelerating frame will have a red/blue shift and a 'gravity' redefining the clocks in your spaceship depending on where they are placed relative the spaceships overall acceleration/direction. But in theory there should be no difference as i think of it. The speed of 'c' is constant in all motion. Maybe someone here knows a better way to define it?
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"light is always propagated in empty space with a definite velocity c that is independent of the state of motion of the emitting body." is perfectly correct, if we assume a propagation, and remember the fact that motion and gravity distort the geometry that 'photon propagate' in, same as the Earth distort our definition of a shortest line, that as we measure it on a bent surface.
Relativistic aberration is about how 'things' move, relative motion. The faster you go, the more acute angle of rain. If you bounce a ball on the floor inside a very fast moving railway car (made of glass) it will to you go straight down and up, but according to your friend on the railway-bank describe a 'V' formed motion as it bounce.
So it is observer dependent, and a function of relative motion, meaning that you can use the same logic for that railway car as we used for the 'light clock', namely that the railway car must present your friend with a 'slower time', relative his clock. According to your friend the ball had 'two directions', not only down but also moving in the direction of the train, and so it also had to take a longer 'time' for it to cover that distance, relative his observation/clock. And you can use a 'light clock' standing vertically to find the same thing happening, the difference being that the light clock uses a 'invariant' bounce, whereas the balls bounce varies with its momentum.
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The strongest objection to this way of looking at it comes from the fact that all uniform motion can be defined as being 'at rest', but the geometry I describe must be true for an accelerated motion too, so to refer it (time dilations and Lorentz contractions) solely to acceleration is not correct, as I see it. To me it is a question of 'energy', and if the universe has a way of defining different energies to different relative motions. And as far as I can see the universe have a definition of that, even though it won't be measurable in a 'black box scenario'.
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If we think of a and b as some locations on a bent surface, represented by 'O', then you have 'a O b', where the 'shortest line' obviously is a tunnel :) but most often easiest to follow the surface. And that's what that 'photon' seems to think too, mostly that is. As you have 'tunneling' there too. And gravity's 'surface'? That would be something to see, 'gravity unwrapped ' I wonder how, and if, it even would be possible to do that?
