1)
watanagashi "style" : this ritual is based on something consecrated to Oyashiro-sama, i believe. also, the previous deaths were aknowledged as "oyashiro-sama curse", whatever the current and true cultprit is. (it means that everything was under oyashiro-sama will : she decide who will die and who will fulfill its wrath)
In watanagashi and Meakashi, hinamizawa people will probably know who is the culprit (either shion or rika herself).
leaving them theses possibility of reasoning : a tragedy occured by some unexplained madness etc. (it is way better explained with shion : she killed every gosanke head. People will probably understand that as a "revenge" since she was in the shadows of her sister. )
But, in the other chapters, they have absolutely no clue who is it (takano is "dead", and used probably a perfect cover via the yamainu, just like she did with her fake death. the irony of it is she didn't need to make a "date matching" corpse, since the people there will be scary about some "undead" and the yamainu manipulating the information (dental inprint, etc))
so, as the time pass, hinamizawa people are kinda screwed : their sacred miko of their almighty god is now dead. the culprit is not found. with some doubt and fear (remember that hinamizawa syndrom expose pretty quickly any paranoid thought), people will explain the matter with the easiest way just like they did with the previous watanagashi incidents :
OYASHIRO-SAMA CURSE !
like Caiobrz interpreted pretty well what i meant with the watanagashi, thoses who managed to survive will think it is all oyashiro-sama will, and they are punished for their fault (rika death etc). the TIPS also show that hinamizawa people are mutilating/suiciding, along with the fear to the nation (though it could be worse if the hinamizawa syndrome was revealed)
2)
if watanagashi doesn't affect anything, the watanagashi factor will simply give takano some credit about the syndrome as well.
if you were a spectator without any clue/tips just like us, only knowing the events during ONE CHAPTER with the disaster (say tatarigoroshi or tsumihoroboshi), who will you suspect with that style of killing?
a fanatic? or a mad researcher?
i'm exaggerating a lot, yes. but i demonstrate that the wantagashi has 2 benefits to takano.
at last, the government trust takano.
they approved the theory and also the plan. but the government has faith in their hidden medical facility to cure the syndrome.
why would takano say they admit the possibility of some syndrom etc in
Yagouchi's Quarry (Piece #10) ?
the government seems to have had a shadow project, but they are also investigating the matter to solve it, before being forced to finish all of this shit with such brutal manner.
or, it would be not logical that they would give such medical facility along... why not just launch a nuke to hinamizawa instead?
indeed, i'm not pretending that my whole theory "takano's queen theory as an excuse / 48h problem / for grandpa love sake and god eccentrical dream" is flawless.
but i think this is the closest one so far from the truth (and it's getting even closer with the new pieces XD)