Georgina
Einstein designating any entity as an observer does not make it an observer. What does so is the receipt of light. Indeed, from the physical perspective, every entity is an observer, because every entity receives light. The waste basket to my left is currently receiving a light based representation of the chair, as indeed is my ear, and my eye. The point is that consequent upon the evolution of sensory systems, the latter entity can utilise what is physically received, the others cannot. But this is irrelevant to physical existence, what existed, existed. Its existence is not a function of being sensed. All that creates is an awareness of the existence.
But the relevant point here is that Einstein has no observational light, nowhere in his narrative or examples does he have light being received. He has disassociated rays of light/lightening/etc. There are two very important implications for this: 1) he has deemed the reality of light to be existent reality, 2) the light which he refers to is just a constant, ie he could have just said 'let us take a constant in order to measure duration and distance'. One can of course reasonably speculate why he chose light, because he failed to understand his mistake of conflating existent reality and the light based representation of it (ie point 1).
Einstein's relativity has nothing to do with appearances, neither was it intended to be so. Neither should any physical theory be based on any aspect of the subsequent processing of what is physically received. Physics is concerned with what is physically received and what created that. Einstein's concept of relativity, which is incorrect, is the function of two counterbalancing mistakes, which is why at one level it 'works', but at another level is causing people to think there is something wrong with it.
Paul