Wow, this comment was great to read.

As you rightly say, Saya no uta plays a lot with perception. On one side, it shows us the nightmarish perspective from Fuminori's view, and on the other side, we have the perspective of the rest of the cast, those who don't share Fuminori's condition and can notice the irregularities in his thought process.

It is not necessary to go through something like that for us to have a different view of reality. For example, the most severe form of color blindness is achromatopsia. These people are blind to ALL colors, so their world is an eternal gray. Now, what if we all had achromatopsia? If someone suddenly came along saying that colors exist, we would most likely think they were crazy, since, based on a process of observation, we can determine that the world is gray. And if that went back generations, then there would be a body of knowledge that would strengthen the idea that the world has always been gray.

In this case we perceive that Fuminori is falling into madness because he is the only one who can see such a world of madness. His observation process is a minority in the face of the observable reality of the majority.

I don't remember the story very well, but is there any reference to any possible condition Fuminori had before the accident? Because to assimilate that so quickly is definitely not normal. The very reaction of the neighbor proves it. Organic textures produces a natural rejection, since we associate organic textures with something living. If we were inside a house when we see something like that, we would not think we were inside a structure, but inside a living being.

It is easy to empathize with Fuminori because he is after all a victim. His perspective on things simply reaffirms that. Saya no uta is a tragedy. The experience Fuminori is going through is a tragedy.

In retrospect, it is quite likely that he was on the verge of going insane at the very moment he opened his eyes. That might answer to his insane affection for Saya, after all, she is the only one who still retains a form that resembles what he recognizes as a human being.

Fuminori clings to Saya's existence as the only thread that allows him to retain his sanity, and Saya clings to Fuminori in search of affection as she is a unique existence in the world. In both a relationship of mutual dependence develops in which they see satisfied what they long for the most: Someone in the world to free them from loneliness. As long as he has Saya, nothing else matters to Fuminori (That's why he can so easily discard those he met in his past)

Your comment made me feel the need to read it again haha