Sunako was mind-controlling Seishin. Thus explaining why he so readily did everything in his power to serve her, and sheltered her by the end of things.
It's chance who comes back as a Shiki and who doesn't......However, it's not wholly random chance. There's probably some baseline probability that people come back as Shiki after being killed by Shiki, no matter their circumstances (thus explaining why innocents like Toru and Ritsuko could become vampires). However, certain things about an individual—which could range from genetics to state of mind—can boost that chance of coming back as a Shiki to well above the baseline, which is why so many of the Shiki we meet are strange or troubled in some way and were when alive.
Jinrou, at least in the anime and manga, are those who willingly gave their blood to vampires from the start.We see this happening with Seishin, and at least in the anime and manga, it's true of Natsuno as well. We don't know enough about Tatsumi or Yoshie to know how they originally became Jinrou, but that means there's nothing to say that their situations were any different.
Masao's Missing Mom died in childbirth.She's curiously absent from the family and nobody ever speaks of her, suggesting that she's been dead for a really long time. Given how much older Masao's brother is than him, and how old his father is in general, his mother would also have been pregnant at an older age, and older women are more likely to die in childbirth than younger women. We also know that medical care around Sotoba isn't airtight, because Ikumi had children who died in infancy.
Sunako is not originally from Japan, or if she is, she's not as old as she's commonly made out to be.In the flashback we see of her as a normal girl, she's wearing western-style clothes and living in a distinctly western-style house. It's true that her name is Japanese, but there is no direct evidence that that's the same name she was given at birth. She might have adopted it somewhere along the line to blend in better in Japan.