Peter
"Yes, I am only too aware you take the pre Galilean view of this"
My view, as you term it, is the definition of referencing. It has nothing to do with pre-Einstein, indeed, he stuck relentlessly to this principle. This is demonstrable in his various definitions of the principle which become more and more generic. For example, here is the last one in SR & GR:
"By application of arbitrary substitutions of the Gauss variables x1, x2, x3, x4, the equations must pass over into equations of the same form; for every transformation (not only the Lorentz transformation) corresponds to the transition of one Gauss co-ordinate system into another". (Einstein 1916 SR & GR section 28)
Or:
"That in general, Laws of Nature are expressed by means of equations which are valid for all co-ordinate systems, that is, which are covariant for all possible transformations". (Einstein, Foundation of GR, 1916, section 3)
The point is that to make any judgement, there must be a reference. That reference can be anything. But once selected, it must be consistently used, otherwise the judgements are not comparable. That is what referencing is.
His point was (or at least Lorentz's point was) that under certain circumstances, matter and light were affected (dimension altered and light no longer travelled in straight lines at a constant speed). And if this was so, then one needed to take account of it when making judgements (ie referencing). The cause of this dimensional alteration also caused a change in momentum. So something (which includes light) that was not moving in a uniform rectilinear and non-rotary motion would be exhibiting the effects. In other words, changing momentum was an indicator that this newly discovered, and really small effect, was occurring.
"Each rest frame IS a local reference for phenomena travelling within that frame"
Yes, it is, that is precisely what I am saying. Because it is then not a reference for the 'next frame'. To include 'the next frame' in your comparison, you must maintain the reference first selected, not keep 'switching references'. Or establish a new reference from which both the first and the next can be referenced against a consistent reference. Furthermore, the concept of 'rest' is irrelevant. The only problem with 'not-at rest' is that, according to them, dimension alteration is occurring. So having recognised that, calibrations can still be effected, they just become more difficult.
"This explains observation"
No it does not. Observation (or indeed any form of sensing) is irrelevant. Physical existence is not affected by sensing. Light is a physically existent phenomenon. It travels. Sometimes it hits a physically existent phenomenon known as eye, most times it does not. The physical circumstances are the same, whether in its travel it hits an eye or a brick wall. It is just that eyes are the front end of a sensory system that can utilise the light, bricks cannot.
"your improved explanation of 'stillness' above"
It was not an improved explanation, I pointed out to you that I used the words "in effect". In the context of maintaining the character count, which I singularly blew by about a page, though I only had 9 pages, a degree of cryptic writing is bound to occur throughout. And the 'stillness' is not inherently the reference point. Physically there is no form of reference that is somehow detached from physical reality, and can therefore be used as some omnipotent reference.
Paul