

All of it is being alleged—that’s what an accusation is.
But they’re not accusing her of arranging sex with boys who were allegedly wearing masks, they’re accusing her of arranging sex with boys who were actually wearing them. In the context of the act of which she’s accused, there were no allegations.
“Alleged” isn’t idempotent—every time you add it, it modifies the meaning.
Take the sentence “Police accused John Doe of inciting a lynch mob to attack the alleged rapist“. The police aren’t alleging that the victim was a rapist, they’re saying the rape allegation was part of the context of their own accusation against John Doe.
If an act is described as an accusation, it’s already implied that everything within the description is an allegation by the accusers. But if something within the description is itself labeled as “alleged”, that nested allegation becomes part of scenario the accusers are reconstructing.