Peter
You start by asserting that you have "no issue with any of your comments", then go on to say something which, as I have already said, is contrary to what I am saying.
1 The essential problem with your theory is that if one reduces the reference against which speed is being calibrated to a singular circumstance, ie each frame, then given the way light propagates, the resulting relative speed will always be the same. This is self-evident, but also fails to address comparability of references, because you are using a different one on each occasion. It is the same form of circumstance, but physically a different circumstance.
What matters here, is the speed, ie duration incurred, for light to travel from A to B. Light being that physical effect which if received, the eye can process. Light is just a physical entity, which, as a result of the evolution of sight, has acquired a functional role as a physically existent, independent representation of what occurred. Leaving aside how, given its physical properties, proficient it is at fulfilling that role, its travel is affected by the physical circumstances it travels in.
2 Leaving aside the correctness of your argument, it has no impact on relativity. Immediately you speak of SR, which as I have pointed out so many times, with quotes from the man himself, is not what you are saying it is. However, leaving that aside and just considering the concept of relativity. The point is that there is no observational light in Einstein. The second postulate, and hence the possible contradiction, is irrelevant. Einstein just used a constant to calibrate distance and duration. It could have been anything. It was not observational light, nobody observes anything in Einstien's writings. There is a 'ray of light', later it is lightening. In Cox & Forshaw it is a light beam clock. c is just a constant. In other words, the attempt to resolve rate of change and light speed constancy is a wild goose chase, the issue was never there. The so-called paradoxes do not exist. Because the man spoke of observers, etc, people have been assuming they were, and that it is all about light. But there was no light.
Einstein did not make any presumptions about background frames, this is your interpretation of what he did. He made two fundamental, and counterbalancing , mistakes;
-he failed to understand how timing operates
-he did not differentiate between what occurs, and the light representation thereof
There is no relativistic effect in existence, it occurs at a specific time in a specific physical state. The point is that light representations of that then travel and are received at different times depending on spatial relationships. The relativity which he attributed to existence (in effect, he did not mean to) is in the receipt of light.
Paul