Eckard,
You're right. Most of the post was cut as I used a symbol that seems to do so!
Not just Einstein; it's a famous problem in all astrophysics to derive a way to measure 1 way speed! And I didn't suggest you employed ether, only that my (cut- but given before) "detailed critical comparison" doesn't either. You didn't open up to allow yourself to grasp it before but maybe now;
Fermions are condensed as pairs FROM some condensate, they are not 'ether'. (We've recently found thousands of time more of this space plasma than believed - i.e. 1014/cm-3 even locally in Earths turbulent ionospheric shock.
We know fermions couple with Electromagnetic signals (including the visible range). We also know such plasma has a refractive index of n=1 (so has NO detectable EM 'signature'). Now all findings support the notion that all particles re-emitting EM energy do so at c. That is LOCAL 'c' IN THE CENTRE OF MASS REST FRAME OF EACH PARTICLE.
Lastly we know that massive clouds of space plasma move around in space (most often around and in the LOCAL rest frame of co-moving bodies). We also find 'birefringence' (TWO apparent paths/speeds) in less dense plasmas, gases and some other media.
I hope that 'nutshell' explanation was adequate.
Best Peter
Now all we have to do is employ our brains analytical functions and put all those findings together logically. To do it for you;
EM propagation is at 'c' locally wherever fermions exist, or at c with respect to the last emitters interacted with. Signals may appear to have 'curved paths' in diffuse media until all is interacted with or where (ubiquitous) particle density gradients exist. Light will therefor travel through OUR Solar system at c wrt the SUN unless near a planet. And light in OTHER solar systems will propagate at c wrt to THAT sun, NOT OURS!!
That description is consistent with ALL findings, including ALL the disparate and otherwise confusing interferometer result. It also solves many anomalous findings. i.e. the KINETIC reverse refraction of light in plasma clouds moving across it's path, (as also found by Lodge in his spinning glass disc experiments).
Now in fact that's very close to your own description, apart from it recognizes the known physics of coupling, and also has that massive power to resolve all anomalies and paradoxes (think of any for me if you like and I'll explain how). It's no more 'one way measurable' than any other model but is both fully consistent AND exceptionally useful.