I think who the detective is might vary by arc. I know Van Dyne's rules mandate the presence of a detective, but I don't think Knox says anything about it. Keiichi is never the detective. Mion and Shion are never the detective. I don't think Rika is the detective in any arc either (except MAYBE???) Minagoroshi-hen. This is just a guess, but the closest person to a detective in novels 1, 3 and 6 is Ooishi, and his role is minor. Akasaka is the detective in the 4th novel. The closest to detective's objectivity in Watanagashi-hen is Rena. Ooishi is also helpful in that arc as well.
On the whole, I'd say Higurashi is about as compliant with the concept of "fair play" mystery as Persona 4; i.e. some violations, but on the whole it seems to try to play fair by providing adequete clues and foreshadowing for the player to find the culprit. Yes, I know Persona 4 snaps Knox's 2nd in half, but the point is the same (the smoking gun in that series being found in October).
As per the theory I mentioned in spoilers for those interested.
Spoiler for spoilers for 8th novel:
Keiichi getting HS renders "blood borne" and "vertical" transmission as unlikely. Ooishi and Akasaka not getting HS rules out the "oral-fecal" route of transmission. The two most likely routes are either aerosol or saliva. IMO, I'd bet on saliva, which is harder to spread than aerosol, which also explains Ooishi and Akasaka not getting HS.
Takano's difficulty in physically finding the parasite in non-advanced stages indicates that an ability to live in a dormant state such as hiding in the host's genome seems to be nessecitated. Indicates that a virus is the most likely causitive agent. DNA or -RNA genome are most likely. However, I suspect that there is a minor research flub on R7's part (or possible creative liberties for "rule of scary") since an electron microscope should be able to detect these parasites. I say minor because R7 is anauthor who's usually quite through in his research. The scenario with Rena is also evidence of a dormant asymptomatic aspect to its life cycle.
I suspect that whatever the parasite is has a relatively low shed rate but an alarming ID50 rate (i.e. the amount of parasites required to successfuly infect 50% of a test population, usually rates, with lower being more dangerous). An act as harmless as sharing food could spread the parasite. If I could pin it on someone to be the vector for Keiichi's infection, I'd bet on either Rena or Mion.
As to how HS wrecks havoc, I suspect the parasite works more through corruption of brain chemistry rather than cell destruction. Namely, the corruption of the dopamine and serotonin pathways. Dopamine overexpression is tied to paranoia, fear, hallucinations of multiple variaties, and heart attacks due to fear. Serotonin malfunction is tied to increased aggression.
Based on all of these things, it is my conclusion that Hinimizawa Syndrome is based off a Borna Virus. Even today, Borna virus is poorly understood. Methods of testing for it in humans are unreliable even today. We don't know if it even infects humans (suspected to do so) or that the difference of severity is due to humans having stronger defensive adaptation against the virus than other species. The virus is suspected to be involved in mental illness.
A bit off topic, but I think we might have a way to find some answers to some of our unresolved questions. Apparently, R7 answers questions on his twitter page. If we have someone who's fluent in Japanese, perhaps we could have someone ask the questions as a representative to the anime suki community. Personally, I would just love to get some answers regarding HS (which I found to be a really cool yet absolutely scientifically plausible solution), and maybe find out if the theory above was correct or not. I wonder how R7 would react to said theory. Maybe even some of the questions that are currently ambiguous in terms of how they're interpreted.