You depicted the results of the experiment as evolving from the source to the detectors again, asking why there is an initial state A of the laser that physically cannot evolve to give the state ARRT + ATRR (detection by the bottom observer).

The fact is that if you look at the possibilities from the point of view of each possible detector (as I explained in the article), you'll find that what you call state A includes light oscillations that are not physically possible.

You think that every time light comes out of the laser, it can reach any of the detectors equally well. But the fact is that the light that comes out of the laser always does so according to the constraints of the experiment, which forbid any detection on the lower path after the second beam splitter. The light does not decide where to go "in flight", it always emerges according to the allowed detection possibilities, even if this means that it cannot reach certain regions. It's not just the laser that decides when to fire, independently of everything else, it's all the elements of the experiment (and actually the whole history of the universe) that constrain (but do not determine) when the laser can actually fire.

So the state A you describe never included waves that could evolve into the ARRT + ATRR state, because that evolution is physically impossible, but not because of anything during that evolution, but by the very nature of the initial state A. You erroneously infer that the evolution of A to ARRT + ATRR should be possible because the bottom observer is placed there, but in fact, every state A of the laser is already constrained by the experimental setup itself to never physically evolve to the ARRT + ATRR state. Due to how we arranged things in the experiment, the bottom path after the second beam splitter is a region of spacetime that does not exist for the photons that actually emerge from the laser.

If the universe consisted only of this experiment, and the only observer in this universe were the bottom observer, the laser would never fire.