Having read many comments about the same thing, I don't think it's so hard to suspend disbelief on this one:
i. It's already been established that the more powerful the ghost/monster/whatever, the less noticeable it is to mediums, being more adept at concealing its presence. This was clearly the case of the boy, because it was powerful enough to defeat one of the miko spirits protecting our protagonist, which is not something the barrel monster in chapter 14.2 could boast of.
ii. I think the scene in page 8 is "Miko's impression of normalcy". The author's drawing what Miko thinks are the living people, and proceeds in the next page to show the monsters she has already pinpointed. Probably the author's intention is to fool us whenever Miko, too, is fooled. Which is different from chapter 9, because Miko obviously knew her father was dead.
I think these extremely powerful ghosts must be quite rare, since Miko has only met two who can disguise as normal humans, and only one who could truly fool her. But yeah, it should work as a wake-up call that ignoring people has its limits. So far, the safest way would be to never acknowledge (by visual contact or gestures) anyone who doesn't talk to her first. Or better yet, try to confirm that other people also can see them. It's a little like the film It follows.
