Edward
Congratulations on a professional analysis and clearly written essay, from an optical physicist/astronomer whose also a fellow Architect. However. I've also been working on a solution to the SR paradoxes, and found a full ontology offering a more simple and consistent solution to your D23 (train, light flash) case.
First I must express agreement with your hypotenuse case, which is also a critical falsification of either the 'falling raindrop' aberration model or CSL, the raindrop photons also not quite matching observation (see my essay).
Bohr almost failed a young Heisenberg's thesis as he didn't know how a lens worked at a quantum level. Most of us still don't. Fresnel's index of refraction n is a constant. No matter what the 'speed' of the dielectric medium wrt it's surroundings light propagates at c/n within it. Your moving medium represents a 'block' of dielectric medium, or a train defined by glass windows, in which the observer is at rest. The outside observer is also at rest in another medium, the air on the embankment.
Whether the flashes are within or outside the trains windows, the light travelling to one or other of the observers must negotiate the glass. Physicist often ignore optical sciences, but at their peril. Light changes speed at the glass, and does c/n in the moving glass frame. It is then re-emitted at the glass surface to do c in the medium it is then propagating within.
You may now see the rest of the jigsaw puzzle falling into place. The c of propagation in the outside air is NOT the same c as within the train. Even if one is a vacuum and the other water, they are NOT the 'same' speed to a single observer in either frame. The dotted circles are therefore different.
Your picture then needs to be a video to capture reality as the train is moving. The outer part of each circle expands in the stationary medium, but the inner is dragged by the train as it expands. Observation IS simultaneous, but the observers are not opposite at the time. A Physicist will not recognise this solution as he is trained not to have the required visualisation skills.
The implications of this are quite massive. The whole consistent ontology covering much greater matters is constructed from epistemological elements observed in my essay, using logical foundations and structure to remove all paradox and anomalies in SR and QM. I hope you'll read and understand it (please forgive the theatre and read and consider more deeply).
I like your essay in any case and wish I could achieve such clarity of description. I'm glad Vladimir mentioned it to me. Please do let me know how you get on with mine and do ask any questions of any areas you may not be familiar with. I'm still trying to develop a more effective means of communication of effects of kinematic evolution. Any advice is more than welcome.
Very best wishes.
Peter
PS I think you may like and understand my last years essay on the similar fundamental theme better, No 10 in the community list. I'm scraping to stay in the top 35 this year.