Even if Toshio were not the leader, he'd still contribute. He doesn't only kill the Shiki because it's his village, but because he doesn't see them as human and he has a strong moral code indicating the murder of humans to be wrong. As the village doctor, it's his responsibility to keep anything from killing the villagers: infection, disease, wounds, etc. This includes villagers killing other villagers. So even if the role of a doctor wasn't also the role of the leader, I think he'd still behave the same way since he's a natural leader. If he wasn't a doctor at all, though, what he'd do then is a more interesting thought. Toshio the Regular Villager would probably still investigate it. I can imagine him finding out that someone is sick (let's say one of his neighbors) and then going out at night and hiding in a bush somewhere by the sick person's house to watch the entrance. After a while he'd see a dead family member of the afflicted person go in.

I remember Sinnesspiel saying that the reason Ozaki was more open to the idea of vampires than Seishin is because as a doctor, he had simply exhausted all other options, which fits well with what you're saying here. For me, it appeared that Ozaki hadn't considered thinking outside the medical box until Natsuno planted the idea. He only started seeing the movings and disappearances as significant when they were evidence to support his theory of it being vampires whereas before when he had been thinking in terms of 'diseases', there's no disease that makes one want to move so he didn't consider them as serious. Although, even if Ozaki had thought they were somehow connected to the epidemic I doubt he would have deduced that it's vampires by himself because there's no vampire lore that shows that vampires make people move away. In other words, if you're already on the idea of vampires it's not hard to deduce that that led to the disappearances and moving away, but if you're not on the idea yet it would be hard to make the connection. So going back to Ozaki embracing the idea of vampires in the first place, at the time the logic leap from 'infectious disease' to 'vampires' was common sense; all the other theories have been exhausted, so moving onto the next one.

Now I'm curious about what you're saying with Seishin going off the deep end post-Shiki; why do you think that? I thought that after the end when he finally leaves Sotoba with Sunako, he's finally at peace.

Thanks for the link! Can't wait to read...there's precious few details about Naru in the canon GH, makes me sad! D;