I think we’re on a similar page for a lot of things, but I want to emphasize that the goal isn’t to ‘solve’ Matsuribayashi, but rather to ‘replace’ it with a solution for the other seven chapters. So built in to the idea of an anti-fantasy solution is a couple premises. If your theory has witches performing magic, it’s not anti-fantasy, right? So are undetectable brain parasites, or drugs that make people rip out their own throats. It’s completely outside 1980s science and technology, so having them in that setting is a fantasy just like Kannon whipping out his light-sabre.
what exactly is the illness (does it exist)?
A solution where the illness exists and Hinamizawa syndrome is an actual thing greater than human stress factors and coincidence would definitely be anti-mystery in my eyes. I could maybe see ‘Takano believed in the disease based on grandpa’s crazy theories and forced evidence to fit that narrative, then convinced other people’ as having a place in anti-fantasy, but the Hinamizawa syndrome itself is definitely a piece for the ‘antie-mystery’ side of the board.
Where does Hanyuu come from?
In an anti-fantasy account, Hanyu would be either Rika’s imaginary friend, or perhaps we could say an alternate personality like the many voices inside Sayo’s head, but that one is pushing at the margins. A firmer anti-fantasy answer has her as an imaginary friend.
Did Takano inject parasitical drugs or was it a type of drug that made the lymph nodes itch and other symptoms to play along into her creation of the Hinamizawa Syndrome to make it “Real” despite it not existing?
The idea of Takano or someone else faking it so that it looks like someone died of a Hinamizawa syndrome is certainly anti-fantasy, but using a drug like that to do so would be anti-mystery. A ‘purer’ anti-fantasy solution might be something like:
Tomitake was tortured by having his fingernails ripped out by some device similar to the one used for Shion’s atonement to the Sonozaki family, then he was murdered by having his throat torn out. Afterwards, the killer arranged the corpse in such a way that it might look like Tomitake ripped his own throat out to a superstitious person, or someone who believed in Hinamizawa syndrome might think that he had succumbed to that disease.