I don't think schizophrenics are inherently narcissistic. Not like Cluster B disorders, at least.
I think schizophrenic grandiosity stems from something else. Bipolars have feelings of grandiosity, too.
As do cocaine users. This might be linked to excess neurotransmitters. Dopamine/noradrenaline? Who knows. They make you feel invincible, you think faster and you go further than most people would deem healthy or reasonable, while high on these neurotransmitters.
Also, some schizos might have great abilities and take everything just a bit too literally and mechanically. I think this fuels their rigidity and arrogance that might come off as narcissism. Not to mention their genuine lack of affect and difficulty relating to others. Again, this is different from NPD. NPDs are more aware and malicious. Think of the NPD/BPD/HPD as someone who is vampirizing others.
The schizophrenic is not focused on his relationship with OTHERS, he is in his own track. He doesn't need to exercise a relationship with someone to assert his grandiosity or make people believe in it. And also, the NPD is aware of it. of how unreal it is. Schizos arent. NPD/BPD/HPD are just selfish and they don't really believe in themselves. They might never deny a lie, but it is not a firm belief within the individual. He can tell lie from truth. And thy're usually extremely insecure and a lot of their behavior is COMPENSATORY. Though the absurd selfishness is there, it's not an "innocent" one like in schizos and bipolars. it's antagonistic. Antagonistic to it's core.
But NPD and schizo disorders will have a lot in common, and superficially might seem the same. Also, individuals might overlap the disorders, even if in a transient manner.
But no doubt, CN, schizos and autistic individuals are tough to deal with. They're stubborn, they're hyperfocused and when not in touch with reality, they simply refuse to believe the truth and go on various different grandious manifestations on their persona. The classic "I am Napoleon Bonaparte" figure on the old mental hospitals is very real. Schizos do feel grandiose sometimes, specially when psychotic. Even if they aren't, the negative symptoms related stuff makes them quite rigid in their thinking styles and how they deal with other people. Schizos are usually quite difficult to relate with.
In a world where A = C, if a schizo decides A = B, no force on Earth will change his mind unless presented with several very precise evidences and pray for his mind to accept it. And if he is losing touch with reality? Lol, self explanatory.
Dopamine whackiness makes a person quite intelligent (for some individuals), but also makes them quite rigid on the way they think and too convinced their minds is correct just because, in their minds, it feels and seems completely, perfectly pinned down.
ADHD people are usually a pit of creativity because their minds make too many free associations. But they have a hard time expressing it because of lack of focus. The schizophrenic mind tends to be the opposite.