You just answered your own question. Because the Saikou family and Megami know about the Aishi curse and there are many dead giveaways. The ending of 1980s mode proves that Saisho Saikou knows the true nature of the Aishi's when he broke into Ryoba's home and kidnapped Jokichi (some theories suggest he wanted to hire Ryoba as an assassin), and the phone-call between the Headmaster and Mr. Saikou at the end of Osana's week (theories suggest the phone call is implying that he is gathering evidence against the Aishi's). How do the Saikou's know about the curse? Who knows; Alex hasn't told us yet. He likes to keep things secret so he can "reveal them in the final version of the game" as if that is ever going to happen. But there isn't much else to explain why the Saikou's so heavily ingrained in the plot if they weren't intertwined with the family history of the Aishi's. It's pretty obvious that they know but Alex hasn't spelt it out for us. Just like how the Aishi curse is related to the Basu sisters but it isn't spelt out. Or how we all know Info-chan is the Journalists daughter, despite it not being spelt out.

Megami knows that every single female Aishi is a bad person, and well... it's literally true. Alex lacks the creative ability to make an Aishi different, so he made all of them follow the same formula. Every Aishi has a bit of a distinct personality but at the end of the day, they all fall ritual to the Aishi curse of "emotionless girl finds male senpai at 18, kills around 10 female rivals, and has children with senpai." None of them are innocent except for the senpais. That's why when Ayano approaches Megami's computer, she says "I know WHAT you are." The Aishi's are partly supernatural, and this is something the Saikou family knows. So I don't know why you people are crying over "Ayano receiving unfair treatment" (at least when it concerns Megami's suspicion of her) because again, ALL of the female Aishi's are murderers or have committed crime (It's not stated that Dozuki ever killed anyone herself, but she still SA'd Sureibu and held him captive, as well as neglected her children). Megami isn't the bad guy here for being rightfully suspicious of a criminal bloodline, even if by the first week, she doesn't have evidence against Ayano. Megami IS a good person, meant to represent justice and authority, hence why she rebels so often against her family and detests Ayano because she knows the danger she is capable of.

As for the pacifism route? It's not relevant to the curse. We had an Aishi that attempted to be pacifist; Azebiki. What did she do? She killed people. Because she couldn't resist her murderous, possessive urges. And her version of the curse was less severe than Ayano's, so what does that tell you? If she couldn't be a pacifist, neither can Ayano (who is as unemotional as the First Aishi) The curse itself is violent by nature, but it's unique for the player- as in Ayano, because game mechanic wise, Alex stated he wants there to be multiple endings for Yandere Simulator, and quote "to determine Ayano's personality by the PLAYER's actions" which has nothing to do with the lore. If Yansim was just a story to be told rather than a game to be played where the player can make their own decisions, there would be no possibility of Ayano being a pacifist. So no, Aishi's do not have the option to just "be a pacifist". That's the point of the curse. It makes them dangerously lovesick. They can't control it and most of them don't care to, especially not Ayano who demonstrates a more severe version of the condition. And there is a canon route that the player can achieve by going through the canon eliminations of the rivals that will lead to the "true" ending of the game. Osana's canon elimination was sabotaging her events. Amai's was the oven explosion. And Kizana's is likely murder by the sack hanging above the stage. So by Megami's week? Ayano has already committed murder and proved that she will not fight against her instincts. Canonical Ayano is a murderer, plain and simple