Paul,
Yes I recall you suggesting that interpretation before. It's probably my fault calling his rejected 'ether' frame a 'background' frame, a word he didn't use. It's then a concept that's easy to miss, and often is.
In fact he spoke and agonised of little else about SR from 1905 right up to his last paper. It may be characterised at the 'preferred 3rd frame', and is the foundation of the 'Relativity of Simultaneity'. It was a key part of his SR addresses and papers, inc. 1905, 1921, 1922, 1923, and right on till 1955.
i.e. In the Gothenberg speech he referred to it as; "...the preference for a single state of motion as in the case of the theory of light with a stationary ether,"
In his 1954 space s in motion with space S, space S of course then DID need such a 'state of motion'. It is these '3rd frames' to which I refer.
But he 'threw out the baby with the bathwater' in assuming that any background frame HAD to be a single "absolute" background frame. It clearly does not. The Earth's ('rotating') rest frame is clearly the background frame for ALL moving trains! and the Sun's heliospheric rest frame is clearly the background frame for all (non rotating) planetary ionosphere's, in the solar system not just Earth's.
There is then a 'hierachy' of inertial systems (frames). he realised this himself; "There are then an infinite number of inertial frames which are in uniform translational motion relative to each other" (background after background ad infinitum) but couldn't rationalise it physically.
Einstein had a good excuse, he did not know of the QV. It's only been inadequate intellect that's prevented better understanding by mainstream science space exploration!
So you can see, that it's only (as we all showed last year) using the wrong 'starting assumptions' and poor thinking that causes theoretical confusion!
Without that realisation I quite see how you thought my proposition had a problem. It would indeed have had! I hope the logical (TFL) hierarchical dynamic is something of a revelation. But don't expect to understand it fully without initial effort. Only the most brilliant seem able to do.