I strongly suspect that the Hinamizawa Syndrome to actually be Oyashiro-Sama herself.
Time is meaningless to beings like Hanyuu so she could easily be the Hinamizawa Syndrome.
Satoko supposedly lacking the Hinamizawa Syndrome would be the result of no longer believing in Oyashiro-Sama as a possessing spirit in the same way Takano didn’t manifest symptoms of the Hinamizawa Syndrome due to thinking that the Virus she kept extracting(simply Demon-saturation in a random Virus) was the source of the Syndrome.
The maggots one feels under one’s neck isn’t the Virus but the Human Host attempting to escape the Oni controlling their Body.
In that case the Hanyuu connected to Rika I.E. Eua’s failure would infact be Hanyuu’s reincarnating Grandchild.
Eua would be Hanyuu’s Daughter born from her broken off Horn.
This would also explain Mion(real name Shion) Sonozaki and Kazuho Kimiyoshi. They are the reincarnations of the founders of the Sonozaki and Kimiyoshi Families who helped the Furude Family found Hinamizawa.
I also suspect the source of the Dark Awakenings to be the Sonozaki Clan Founder due to the Eye Color resembling Mion’s Grandmother back before she was an old woman.
I suspect that Satoko injecting Mion with the Hinamizawa Syndrome awoke Sonozaki’s memories and she after discovering that Oyashiro-Sama herself had started the curse had taken over as Game Master of Higurashi When They Cry Gou/Sotsu in order to push Oyashiro-Sama out of Satoko into the Sea of Fragments(in otherwords initiate L6 Hinamizawa Syndrome: the Exorcism-Stage).
Of course Oyashiro-Sama still lacked her Memories so Sonozaki began a plan to restore Oyashiro-Sama while using Rika: The Tsukuyami. The Tsukuyami would continue to attack Rika again and again allowing her to accumulate memories to manifest a vessel for Hanyuu upon which Sonozaki would grab it and hollow it out for the Real Hanyuu to possess.
The next order of business would be to drain & filter the Real Hanyuu from Oyashiro Satoko who was forced to seek out Mion for a currently undisclosed reason which I suspect to be thus: to leech the Sonozaki inside her.
Since the Sonozaki Family is the one that enforced the Village’s worship of Oyashiro-Sama that means that Sonozaki is a lower ranking member of the Jedha Clan of Oni so if Oyashiro-Satoko were to leech Sonozaki from Mion it would work and the drained Oyashiro-Satoko would become a fully new member of the Jedha Clan for the sake of resurrecting the Real Hanyuu.
Now that leaves Kazuho. Her assassination came shortly after a Rika and Hanyuu Demon showed up. I suspect those Demons are members of the Kimiyoshi Clan of Demons that spawned from Rika & Hanyuu’s close proximity to Kazuho.
I’m sure that Tamurahime No Mikoto’s memory loss is the result of the Kimiyoshi Tamurahime taking them with her when she spawned. Tamurahime no Mikoto now has Sonozaki-infusion placed inside her by Sonozaki herself.
I’m sure the nature of the Sonozaki Clan will first be revealed in Higurashi When They Cry Meguri Manga followed by the reveal of the Kimiyoshi Clan in Higurashi When They Cry Mei which will set up Umineko When They Cry’s Sequel which will have the Kimiyoshi Clan capitalize on the destruction of Rokkenjima’s local shrine to infest the Island.
In otherwords the new Umineko When They Cry will have Demons be real and active members of the Kimiyoshi and Furude Families. Since Beatrice is an incarnation of Lambdadelta AKA Oyashiro-Sama that means that Oyashiro-Beatrice will manifest with Hanyuu’s Horns as will Oyashiro-Maria.
The new Umineko When They Cry will descend into a war between Beatrice’s Human Self’s goals(making Battler understand the Truth) and the Demon & Oni Clans’ desire to increase their numbers.